residential solar pv design 101

7/30/2019 Residential Solar PV Design 101

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Residential Solar PV Design 101

What is in this guide?

In this guide you will learn

1. Solar Basics: Power, Energy, Current, Resistance, Circuits,Irradiance, Irradiation, Azmith, Horizontal Tilt, Declincation, Voc,Vmp, Imp, Isc, Temperature and Voltage Relationship, Irradianceand Current Relationship

2. How to Size an Array based customer constraints and siteconstraints and how to estimate power production.

3. How to Design and Size Grid-Connected Solar PV Inverter, SizeStrings and Size Conductor

Who is the guide for?

This guide is for electrical contractors, roofing contractors, generalcontractors, engineers, managers, salespeople, career changers oranyone who is looking to enter the solar PV industry and needs a basictechnical understanding of how the technology works.

Who is HeatSpring?Heatspring Learning Institute provides world class industry certifiedtraining to building professionals interesting in geothermal heat pumps,solar photovoltaic, solar thermal and energy auditing. We have trainedover 4,500 professionals since 2007.

What is HeatSpring Magazine?HeatSpring Magazine is a trade magazine that provides tips, information,and resources to all professionals interested in the marketing, sales,design and installation of geothermal heat pumps, solar pv, solar thermaland energy efficiency.


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HeatSpring Magazine: http://blog.heatspring.comHeatSpring Solar Training:http://www.heatspring.com/categories/solar-photovoltaic-pv-training-courses

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About the Author

Chris Williams is the Chief Marketing Officer at HeatSpringMagazine.

He writes at Cleantechies, Alternative Energy Stocks andRenewable Energy World. He's a clean energy jack-of-alltrades. He has installed over 300kW of solar PV systems,

tens of residential and commercial solar hot water systemsand 50 tons of geothermal equipment. Chris is an IGSPHA

Certified Geothermal Installer and will be sitting for hisNABCEP in September.

If you have any questions.If you read this guide and have any questions or want to get more article,whitepapers or information there are a few ways to keep in touch.

Subscribe to HeatSpring Magazine with your RSS subscribers.

Ask HeatSpring a question on our facebook page.

Ask me, Chris Williams, @topherwilliams, a question on twitter.

Subscribe to HeatSpring TV, our video podcast toget updates on interviews with industry experts on best practices.

Join our linkedin group to connect with HeatSpring alumni and

other professionals.Email. You can email directly: [email protected]. If you have an in-depth question call me at 917 767 820


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Table of Contents

1.SOLARPVTHEBASICS ..................................................................................................................... 4CALCULATINGANDCORRECTINGSOLARRESOURCE ..........................................................................................8

PART2HOWTODESIGNASOLARPVARRAYSIZEANDESTIMATEPOWER

PRODUCTION ........................................................................................................................................161.CUSTOMERCONSTRAINTS................................................................................................................................ 172.SITECONSTRAINTS............................................................................................................................................ 20ROOFCHARACTERISTICSTOCONSIDERANDGATHER .................................................................................... 20COMMERCIALCONSIDERATIONS ......................................................................................................................... 23REMEMBERTOCOLLECTFROMASITEVISIT: .................................................................................................... 26

EXAMPLE:HOUSTON,TEXASHOUSE ................................................................................................................. 263.DETERMININGLOCATIONIRRADIANCE ......................................................................................................... 361.CITYIRRADIATION............................................................................................................................................. 362.ADJUSTCITYIRRADIATIONFORROOFIRRADIATIONANDESTIMATINGPOWERPRODUCTION. ....... 37CONCLUSIONSONPOWERPRODUCTION............................................................................................................ 40

PART3HOWTODESIGNASOLARPVINVERTER,SIZESTRINGS,ANDSIZE

CONDUCTORS........................................................................................................................................421.INVERTSIZINGANDSELECTING...................................................................................................................... 422L OWMANYMODULESCANWEFITONTHEROOF? ................................................................................... 453.HOWDOWESIZETHESTRINGS?..................................................................................................................... 46WHATISTHEMAXIMUMVOLTAGEALLOWEDFORTHESYSTEM?HOWMANYMODULESWECAN

CONNECTINSERIES?..............................................................................................................................................47

HOWDOWECALCULATETHEMINIMUMNUMBEROFMODULESINASTRING?........................................... 494.HOWDOWESIZECONDUCTORS?.................................................................................................................... 521.STANDARDAMPACITYTABLESBASEDONCONTINUOUSORMAXIMUMCURRENT. ...............................542.DERATINGWIREFORCONDITIONSOFUSE .................................................................................................. 551.STANDARDCONDUCTORAMPACITY ..............................................................................................................552.ADJUSTEDFORCONDUITFILL......................................................................................................................... 563.ADJUSTINGFORTEMPERATURERATING ...................................................................................................... 57

ABOUTTHEAUTHOR ....... ........ ....... ........ ....... ........ ....... ........ ....... ........ ....... ........ ....... ........ ....... ........ .61


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1.SolarPVTheBasics

Weve created tutorials on solar thermal design 101 and geothermal design, but

we havent paid the same attention to solar PV yet.

This is the first in a series of posts well publish on the basics of designing and

installing residential solar PV systems. The goal of the series will be to get the

basics covered. If youre an experienced installer, none of this information will be

new to you. If youre brand new to solar, it will be helpful. But keep in mind, we

ll

be skimming the surface. Please leave any and all questions in the comments

section and Ill address them.

Were going to begin with the basic terms. This is very important for design

because you need to understand the concepts before you start applying real

numbers to a design. It will also help with sales because it will help you explain

some basic terms to curious customers.

Power

Power is an AMOUNT of energy. Its the measurement of energy, measured in

kilowatts (kW). Power is measured in an instant. Most of the sizing done in solar

PV design; conductors, inverters, fuses, the size of the solar rates is based on

how much power will be passing through a specific component of the system.

Because power is measured in an instant, it can vary widely over time and from

minute to minute.

Power (watts) = current (Amps) X voltage (volts)

Energy

Energy is the is the actual work done by power. It is measure in kilowatt-hours

(kWh). Consumers pay for kWh. Its a measure of power over time.


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Power (kW) X Time (hours) = Energy (kWh)

Current

Electricity is the flow of negatively charged electrons. The current is the amount

of negatively charged electrons in a specific part of a circuit.

Many people find it useful to use a water analogy when discussing electrical

terms. In the water example, its useful to think of a dam with a pipe at the bottom

that water can flow out of. The amount of water that can pass through a slice of

the pipe, in other words the area of the cross section of the pipe, is analogous toelectric current.

Voltage

Voltage is a measure of the force or pressure of the electric current in a circuit.

Its measured in volts. Electrons of the same material WANT to

be homogeneous, i.e. they want to be evenly spread out. Thus, if one area has

less electrons then another, the electrons will move in an attempt to equalize.

This flow is what created a voltage potential and causes electrons to move.

To use the water example with a dam. If the size of the pipe at the bottom of a

dam is a measure of current, the height of the dam is a measure of voltage. The

higher the water is one on side of the dam versus the other, the more pressure

there is.

Resistance


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Electrical Resistance is the resistance of the flow of electricity through a

conductor. It does not reduce the current flow of electrons (how many electrons

there are in the circuit) but it does reduce the voltage (how fast they re going,

remember the dam example). It is measured in ohms.

Voltage Drop (volts) = Current (amps) X Resistance (ohms)

Series Circuit

A series circuit is when one negative and positive of each power source or

appliance, are connected together.

Remember, CURRENT is constant and Voltage ADDS in series circuits.

Parallel Circuit

In a parallel circuit, all of the positives are connected together and the negativeare connected together, each separate.

In parallel circuits, CURRENT ADDS and voltage stays constant.


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AC Current

AC refers to alternating current. It refers to electrical systems where the voltage

and current are constantly changing between positive and negative. A complete

cycle is completed when when the current reaches returns to either the peak, or

trough of the wave. Frequency is measured in Hertz (Hz) and is measured in

number of cycles per second. The power in the US is operated at 60 Hz.

DC Current


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DC means direct current. DC is the type of electricity where the voltage and

current stay constant over time. Typical DC applications are batteries, solar

modules, and wind turbines.

Calculating and Correcting Solar Resource

Irradiance

Irradiance is the amount of solar radiation falling on a particular area at any given

time. It is a RATE. Its a measure of POWER, in that its an instantaneous term

that does not consider time. Remember the difference between power and

energy.

It is measured in watts per square meter.

Irradiation

Irradiation is a measure of solar energy, the amount of irradiance that falls on a

location over time.

Irradiation is measured in kWh / square meter / day.


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Irradiation was formerly called insolation.

Solar Energy in the US

The below pictures shows that amount of solar irradiation that falls on the various

surfaces across the US depending on average local weather circumstances.

Horizontal Tilt


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The tilt angle from the sun is the angle from the horizon to the sun. Solar PV

modules will produce the most energy when the sun is shining directly onto them,

from a 90 degree angle. Thus, all else equal, for fixed PV modules the best tilt

angle will be the same as the latitude of the site. For example, if the PV site is at

44 N, the best tilt will be 44 degrees. However, most roofs and and commercial

racking are not at 44 degrees, so you must apply correction factors for projects

that are not at perfect tilts. We will discuss this in a later article.

Azimuth

The azimuth is the number of degrees from true south that the sun, or another

object, is facing. Its used when designing a solar PV system because due south

will provide the best production, all else equal, over the course of a year. Were

not going to get into tracking systems in this series so all of our arrays will be

fixed. However, if the object is not directly south, you will need to apply correction

factors that we will get to in later articles.

Magnetic Declination

Keep in mind that if youre doing site visits with a magnetic compass you will

need to correct your magnetic readings to find truth south. The process is simple.

Determine your declination by look at diagram like the one below and

determining your location.


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If youre location has a eastern declination, youll need to add the numbers to

reading. If from the west, subtract.

EAST Subtract. If youre compass reading was 190 degrees and you lived in San

Francisco, about 17 degrees east, you would need to subtract 17 degrees to find

true south. Youre TRUE SOUTH reading is 173 degrees.

WEST Ad. If you live in Belfast, Maine (about 19 degrees west) and your

compass reading was 165 degrees, you would need to subtract 19 degrees to

get TRUE SOUTH of 184 degrees.

Solar Module Terms: The below terms are terms you will need to understand

when sizing your system.


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Voc: Volts open circuit is the maximum voltage a solar module can ever make

when it has no load on it. Voc is used when sizing solar arrays along with

temperature coefficients to determine worst case voltage scenarios.

Vmp: Volts maximum power is the reading of the maximum volts a module can

produce when under load under standing testing condition, STC, irradiance levels

(1000 W / M2) . If you look at the below curve, the Vmp would be somewhere in

curve on the right in the bend. It will be on the place in the curve the creates the

most power (volts times amps). The number is actually rather to difficult to

calculate exactly and can change rapidly from second to second as the current

changes.

Isc: Amps short circuits it the maximum amount of amps that a solar module

could produce. You will find Isc on the x axis of the above graph where there is

no voltage and thus no power being produced.


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Imp: Amps max power, like volts max power, is the current point on the power

curve when the module is producing maximum power.

Youll find the above material on the back of every individual solar PV module

and it is standard information that manufacturers and distributors will tell about

their product. Below is a product description for two Sharp modules from AEE

Solar. All the data is public and available on AEEs website.

Temperature and Voltage: Its important to understand the relationship between

temperature and voltage in solar modules for design purposes. Whiletemperature does have a slight impact on current, its considered to be negligible.

However, temperature has a large impact on voltage. When you are determining

the maximum number of solar modules in a string, based on the inverters

acceptable voltage window, you will need to take into account expected lowest

temperature ranges that can increase voltage.


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Irradiance and Current:

Irradiance and current also have a direct relationship. The amount of irradiancefalling on a solar PV module will directly impact the current that module is

producing. This is key for understand when performing designs, and

troubleshooting systems.


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This is it for basics. The next post will be on sizing the array based on the

customers needs.

Please let me know if you have any questions or if I was unclear about any ofthese terms.


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Part2HowtoDesignaSolarPVArraySizeandEstimatePower

Production

This is the 2nd article in a series about how to design solar PV projects. We

started with solar 101, the basics. If youre brand new or need to brush up on the

basics, please read it first. It discusses electrical theory, key solar terms needed

to design any system and the relationship between irradiance, temperature,

amperage and voltage among other things.

This section is dedicated to sizing an array based on customer needs and site

characteristics it also discusses estimating power production. The main focus is

residential applications, but Ill also highlight slight differences in commercial

projects.

The goal of the article is to provide a basic process for you to understand how to

size an array and provide you with further resources youll need to continue your

learning. There will be some overlap in this discussion with more advanced

topics, like string and conductor sizing that will be covered in future articles, and

how the design will impact the financial returns of a system, which will be

discussed in a future article on Solar PV financing. If you need to read on up

renewable energy finance, you can start with Finance 101 for Renewable Energy

Professionals.

First, let me outline what well talk about, then I will go into each part with more

detail and depth.

Below is the process for designing a solar PV array.

In the field, most of the power production estimating is done with software.However, Id argue that its still important to understand the theory behind power

production estimates and the variables that impact power production so you can

make sure to gather the correct information when performing a site visit.


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1. Customer Constraints. What about a specific customer will impact the size of

an array? The most common restraints are:

Energy Usage Client Budget

2. Site Constraints. What about the client site will limit array size? These are

the most common details about a site you need to gather and well discuss how

these variables impact the size of an array:

Local Shading Horizontal Shading Available Roof Space and Roof Characteristics (dimensions, tilt, azimuth)

Module Size and Racking Considerations3. Determining Irradiation. In order to compute power production, you need to

understand how much energy is hitting your specific area.

Measured in kWh/M2/day or Sun hours per day4. Estimating power production based on irradiation, customer constraints,

and site characteristics.

Sun hours per day adjust for site characteristics Power production estimates based on solar resource and the amount of modules

you can fit on the roof.

1. Customer Constraints.

A. Energy Usage

A possible constraint on the size of a solar project is the client s energy usage.

Because of how net-metering programs are set up, typically it does not make

sense to produce more then 100% of a clients annual energy usage. However,

because most property owners use so much power, and the power density ofsolar PV is so low, its rare to have an array that can produce 100% of the power

with solar power. Its typical that the solar fraction of a project (total power used /

power supplied by solar) is less then 30%.


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Commercial Considerations

For a commercial client you will need to understand their demand charges and

usage charges. In order to understand if the solar array will reduce their demand

charges you need to understand the load profile of the building and when exactly

their demand is the highest to see if solar will shave that demand. For example,

do they have the highest amount of demand in the summer or winter? What time

of day, early morning, afternoon, evening?

We will not go into depth on demand charges for this post. However, WE WILL

discuss the impact of different electric rates, demand and usage charges in thesolar PV financing article because its critical to understand the value of the

power that a solar project produces. Right now, were just concerned with pure

design.

If you need to learn more about what demand charges are, Ive found these are

good resources:

Understanding demand charges Demand Charges Explained

What you need to collect about energy usage:

Yearly average kWh used by the client Cost of power The value of a kWh of solar is directly related to the cost of the power it offsets.

On a site visit make sure to get a few months of electric bills.

Example

Lets assume a customer uses lives in Houston, TX and uses 550 kWh of AC

power on average per month and wants a solar system that will produce 100% of

the power they use in a year. How large would you need to design the system?

You need to reverse engineer the problem, heres how:


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1. 550 kWh/month / 30 days per month = 18.33 kWh per day2. Calculate and Adjust Irradiation based on site characteristics. According to PV

Watts, Houston gets an average of 4.79 sun hours per day. For now, let s

assume the roof is directly south and at 30 degrees (the latitude of Houston) so it

can harvest 100% of the 4.79 sun hours per day. See section 4 for how we

adjust irradiation based on a roofs characteristics

3. 18.33 kWh per day / 4.79 adjusted sun hours per day in Houston = 3.83 kW ACneeded in production. Now we need to convert to DC

4. 3.83 kW AC / 80% (to make up for the inefficiency of converting to DC to AC.80% is a rule of thumb. You will read more about this in the next part of this

series when we talk about string and conductor selection, inverter selection and

derating) = 4.78kW DCIf the customer wanted to produce 100% of their power from solar energy in

Houston and they had a perfect roof, they would need a 4.78kW DC system.

Well discuss what happens if there roof is not perfect below.

B. Customer Budget

One of the most common client constraints is budget for the system, if they are

purchasing with cash. If they are leasing the system, this will not be so much of

an issue. Learn more about solar leases, prepaid leases and how to sell a solar

lease here.

If your installed cost is $5.00/watt, a 4.78 kW system will cost you $23,900. If the

customers budgets is only $15,000, you could only install a 3 kW DC system.

Things to remember:

Know if its a cash or lease sale. Learn more about lease sales in our free courseabout solar lease.

If its a cash customer, make sure you understand what their budget is. Makesure you understand if they are purchasing cash, or with a home equity line ofcredit or wrapped into a mortgage for new construction.


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2. Site Constraints

Site constraints are the second most common attribute that limit the size of a

solar array, behind a customers budget. Answering the question how many

panels can fit on the roof is a major limiting factor of a project. However,

remember that its not just how many panels can you physically fit on the roof, but

how many can be on the roof and produce maximum power.

**NOTE: Im not going over structural aspects in this part of the series and that

will be discussed in a future post. Remember, simply becasue there is room on

the roof doesnt mean you can install solar. The roof needs to be able to hold the

additional load.

Roof Characteristics to Consider and Gather

Total Roof Area: When performing a residential or commercial site visit its goodpractice to measure the whole plane on the roof where you plan to install the

array, then begin to work backwards and eliminate space that is shaded or

unsuitable for panels.

Local Shading. Local shading is shading that occurs on the roof. Commonexamples include: chimneys, stink pipes, eaves, shading from another part of theroof.

A good rule of thumb for local shading is dont place modules anywhere that iscloser then 3x the height of the obstacle from the object. If a stink pipe is 12

inches, dont place any module north, east or west of it closer then 36 inches

away. You can still place module south of the local shading areas.

When doing a site visit make sure to mark the locations of all local shadingelements. Also, note if there is an attic or cathedral ceilings. If an attic,

sometimes pipes and other items can be moved easily.

Horizontal Shading. Horizontal shading is most often caused trees, but can alsobe from buildings. It is shading that occurs off the roof that impacts the amount of

irradiance hitting the roof.


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Its best to have no shading between the hours of 9am and 3pm for the wholeyear. If this is the case, you will not need to adjust your irradiation numbers for

shading.

If you have any shading between 9am and 3pm during any point in the year youwill need to adjust the irradiation numbers that we will discuss step 4.

Here are two examples of a nearly perfect roof and a roof with some shading.The solar access percentage is what we care about, and this is the number that

will adjust irradiation values. This percentage is a measure of the amount of sun

light youve lost due to shading. If its 95%, youve lose 5% production from the

best case scenario due to shading.

Key to remember: Trees Grow. If youre building an area that has some shading,when you perform your power production estimates it will be good to assumeyour shading will increase by a small amount each year, lets say .5%.

Key to remember: Some states have rebate programs that say a roof must solaraccess of at least 80%.

A great roof: On average this roof only loses 4% product due to shading


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An okay roof: This roof will lose 20% product due to shading.


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Commercial Considerations

Commercial projects seem more open then residential applications because you

can orient the modules how you wish, but there some considerations that are

more critical to watch for on commercial projects:

Local shading becomes much more important. Make sure to havea DETAILED roof plan that shows the dimensions of the roof, and everything else

on the roof that will impact where you can place modules; drains, the footprintAND HEIGHT of the AC units, skylights, height of knee walls, and all other

equipment.


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Examples below of skylights, knee-walls, AC units, and existing conduit.


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Edge of the roof. 6 feet from the edge is common Double check with your fire department about array layout. Fire AHJs are

becoming more and more stringent with where modules can be placed because

they will need access to the roof in the case of a fire.

Remember to collect from a site visit:

1. Raw roof dimensions2. Location and height of all other obstacles3. Shading analysis with a Sun Eye4. Tilt of the roof if residential. If commercial, this will be based on the racking you

use

5. Azimuth of the building. This means, where is the building facing. Its best for theroof to be facing directly south. On residential roofs, you tend to not have a

choice. On commercial, you have more freedom to point the array where you

wish.

Example: Houston, Texas House

The process for determining how many modules can fit onto a residential roof are

the following.


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1. Measure the raw roof. This is an example house in Houston, TX.


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2. Locate all other obstacles. The above roof is perfect, but let s assume that

there is a chimney on the top left of the roof.


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3. Perform a shading analysis. Mark any areas that have less then 80% solar

access. The above roof does not have any shading, but if there was a tree on the

left hand side you would need to get on the roof and use a sun eye to determine

how far the shading goes onto the roof. Mark the section of the roof where the

shading stops!

4. Determine the unusable space created by local obstacles and shading on the

roof. Remember to use 3x the height of the obstalces as the closest distance a

module should be to said obstacle.


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5. Determine how many modules can fit in the adjusted usable space based on

the size of the module and racking.

Youll need 3 things

1. The amount of usable space on your roof2. The dimensions of your module3. Needed space for racking

A few other key tips to keep in mind.


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Its good to make sure the modules do not overhang the ridge. Its good for thespace between the ridge of the roof and the array and the bottom to be equal, if

possible.

It looks best if you can space the array on both sides equally as well, butsometimes this is not possible.

Rectangles, including squares, always look the best. Remember Unirac racking will take 1 inches between all modules but not the top,

bottom or either side. Prosolar is also very common. Other brands are coming

along including ZEP Solar and other brand specific raking, like Westinghouse

Solar. Just know your racking dimensions.

Here is the module were going to use:


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6. Result: 20 Modules Will Fit on the Roof.

Height: The height of the array is 119 inches (59 X 2 + 1 inches for the racking)

Width of the top row: 279 inches (39 inches wide X 7 modules + 6 inches for

each space)

Width of the bottom row: 519 inches (39 inches wide X 13 modules + 12 inches

for spacing)


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This may not be the exact amount of modules for the final design depending on

what our string sizing calculations comes out as OR if we choose to use micro-

inverters or an AC module. But you get the idea of the process.

7. Gathering Roof Characteristics

The two other things you need to collect about the roof that will be needed for

power production estimates are the tilt roof and its azmith. We will discuss power

production estimates next.

The tilt of our sample roof is 30 degrees, or a 7 pitch.

The true azimuth of the building is 132 degrees. The magnetic reading of where

the building was facing was 140 degrees. HOWEVER, we must adjust magnetic

south to true south. Houston has a declination of 8 degrees EAST. EAST

Subtracts, you remember that.

140 degrees magnetic 8 degrees declination east = 132 degrees.


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3. Determining Location Irradiance

Now that we understand the basic process for determine how many modules can

fit on the roof, collecting data about shading, and where the roof is facing.

Heres the general process.

1. Determine the amount of sun falling in your city2. Determine how much of that sun is falling on your specific roof3. Determine how much sun falling on the roof the modules can harvest, based on

how many modules you have and their power rating.

1. City Irradiation.

This is not an official term but its how I think about it. First what were looking for

is how much sun, on average, is falling in the city where my roof is located. What

youre looking for is called IRRADIATION, formerly called Insolation with an o.

Here are some good resources to look up the irradiation in your city:

Whole Solar Sun Hours Map PV Watts

REMEMBER, an easier way of thinking about the term kWh / M2 / day is Sun

Hours Per Day Or how many hours of direct sunlight (at STC) are falling. The

reason I like sun hours per day is it makes calculating power make more sense to

me. If I have a 1 kW array that gets 5 sun hours, Ive produced 5kW (1kW X 5

hours)

According to PV Watts, Houston gets an average of 4.79 Sun Hours per day.


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2. Adjust City Irradiation for Roof Irradiation and Estimating Power

Production.


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In order to calculate the irradiation that falls onto the roof we need to correct the

local information for the conditions of the specific roof. If you remember from

solar design 101, solar modules are most efficient (produce the most power)

when they are perpendicular to the sun. Note, I wont be discussing tracking

arrays in this article. Here are the best conditions for a fixed tilt array.

Azimuth = Directly South at 180 degrees. Only in the northern hemisphere Tilt Angle = Latitude of the Site. Houstons latitude is 28 degrees north, so 28

degrees is the best tilt of the roof.

If the array has a different tilt and azimuth then from the above, we need to adjust

the city irradiation numbers to get an accurate power product estimation for thespecific roof. Here is an example of a table used for locations that are 30 degree

north.

Notice from the above graph that at 180 degrees south and 30 degrees tilt angle,

the correction factor is 1, or 100%. Its useful to analyze this graph to get an

understanding of the implications of different site conditions. This is useful for

marketing purposes to determine good sites from bad sites.


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If the building was facing directly WEST, it would only lose 17%, but if it facesdirectly EAST, it will lose 22%.

Also note what happens when the module is at 0 degrees, flat, it only loses 13%.Mainly due to the fact that Houston is close to the equator so the summers are

long.

Solmetic also has an amazing tool that will tell you the optimal tilt and azimuth for

a building in a specific location. Then you input the specific characteristics of your

roof and it will tell you how much to adjust your irradiation numbers by.

This is data for Houston

Here is a link to the Solmetric tool


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According to Solmetric, the optimal tilt for Houston is 28 degrees, the azmith is

178 degrees. You can find this at the top of the graph.

If you look at the bottom, you can find our roofs characteristics, is says that a

roof with a tilt of 30 degrees tilt at 148 degrees will get access to 97.8% of the

sun.

Example with roof adjusted irradiation

Multiply Houston Irradiation, 4.79, by the roof correction factor 97.8% to equal

4.68 sun hours per day.

We would then use the roof adjusted irradiation numbers in our power production

estimates. For the amount of module that fit can fit on the roof, 20 in hour case.

Note that 20 is not taking into account customer budget.

1. 20 modules X 205 watts per module (find this on the modules specs) = 4100watts DC rated power

2. 4,100 watts X 4.68 average sun hours per day (roof adjust irradiation) = 19, 188kWh DC produced per day on average.

3. 19.18 kWh DC X 80% (to convert from DC to AC) = 15,350 watts-hours ACaverage daily production

4. 15.35kWh per day X 30 days per month = 460 kWh AC production per month.Conclusions on Power Production

Thats a step-by-step guide for sizing a solar array and estimating power

production. The process is slightly different and there is more to consider for light

commercial applications. I will dedicate a specific post to commercial array sizingand power production in the future.

To wrap up what we discussed.


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1. Client specific constraints: budget and energy usage2. How a roofs constraints impact a solar arrays size: Local and horizontal

shading, roof dimensions

3. How to determine and adjust irradiation numbers based on the roof scharacteristics; tilt and azimuth.

4. How to estimate power production based on the irradiation reaching a roof andthe number of modules on it.

In this article, we used a rule of thumb 80% derate factor to convert from DC to

AC. In the next article, we will dive deeper into inverter sizing, string sizing and

conductor sizing, all of which will directly impact this 80% number.

If you have any questions or comments, please leave them in the commentstream.


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Part3HowtoDesignaSolarPVInverter,SizeStrings,andSize

Conductors

This is the 3 part in a series on residential solar PV design. The goal is to provide

a solid foundation for new system designers and installers.

The goal of the article is to convey the basic process for sizing an inverter,

strings and the conductors. You may not be an expert at the end of the post, but

youll have a better understanding of how to do these things.

As always, having specific numbers is the most useful for examples so well

continue with the example from part 2 on sizing an array and estimate power

production. The house was located in Houston, TX and the roof, given local

shading conditions, has enough room on the roof for 20, 205 watt modules.

(see part 2 to see how we got this number)

Here is the spec sheet on the Sanyo HIT 205 module well use for the example.

So, the largest possible size of the array we can fit on the roof at STC is 4,100

watts. We can go lower then this, but not higher.

1. Invert Sizing and Selecting

Given that we know how many modules can fit on the roof, how do we use this

data to size the inverter? The size of the inverter is driven by answering 2

questions:

1 What is the capacity of the existing electrical service?

Per NEC 690.64B2 (2008) 705.12 D2 (2011), an existing electrical service is only

allowed to backfed up to 120% of the rated capacity.


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What does this mean with a typical home?

100 amp service X 20% = 20 amp backfed breaker allowed

20 amp X 80% (for continuous load, we ll talk about this below) = 16 amp

continuous inverter output current

16 amps X 240 volts (or 208 volts, depending on the homes location) =

3840 watts. This is the maximum allowed AC power output of the inverter.

There are a few ways of getting around this, by upgrading the service, performing

a line-side tap, and it can sometimes be accomplished with subpanels. However,for this example, lets keep it simple.

If the existing service only had room for a 20amp breaker, we would not be able

to have an inverter that has a rated AC continuous output that would exceed the

16 amp (see example above) or 3840 watts AC.

Per NEC 690.8 A3 the maximum AC ouput current from an inverter is defined as

the manufacturers continued rated output current.

Max Current (inverter AC circuits) = continuous current output.

For our example, well assume that the existing electrical service can supply an

additional 25 amp back-fed breaker, 20 amps continuous allowed. This limits our

choice of inverter to either a PVI 3000 or PVI 4000 inverter based on the

electrical service capacity, as the PVI 5000 has a continue output current at 208

VAC of 20.7 amps.


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Figure 1 A Sampling of Solectria Residential Inverter Specs


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2 How many modules can we fit on the roof?

From our example, we know that we can fit 20, 205 watt Sanyo modules on the

roof.

Here is the spec sheet for the module

Figure 2 Spec sheet for Sanyo 205 Module


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First, we need to guess the size of the inverter. It s a good rule of thumb to size

the inverter, based on the rated AC continuous output, to be 80% smaller then

the rated STC output of the array. The reason for this is that there is a lot of

inefficiency from the array to the inverter, so if we undersize the inverter, the

array is more likely to hitting the upper limit of the input ranges of the inverter and

will more likely be operating within the MPPT operating range of the inverter.

For example, for our array size at 4,100 watts DC STC, we ve guessed that the

inverter would have a AC continuous output range of 80% of 4.1kW, or 3,280

watts AC.

Youll notice that the naming of Solectria inverters (PVI 3000, 4000, 5000) also

seem to match this relationship between the DC rated power of an array (thename of the inverter) and the AC continuous output of the inverter (2700W,

3400W, 4300W, respectively)

We will choose the Solectria PVI 4000 for our example from our choices between

the PVI 3000 and 4000

3. How do we size the strings?

Right now, we have concluded two things. First, the inverter wed like to use the

PVI 4000 based on the number of modules that can fit on the roof and how their

capacity relates to the inverter. Second, we know the number of max modules

we can fit on the roof. Now, we must begin string sizing.

String sizing is the number of modules that we will connect in series and parallel

before connecting them to the inverter. The size of our strings will determine the

voltage and amperage that is inputted into the inverter.

When string sizing, our goals are:

1. Make sure we NEVER supply the inverter with too much voltage, which will kill it> Maximum string length


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2. Make sure that we can ALWAYS supply the inverter with enough voltage to turnon, given the array is receiving full sun > Minimum string length

What is the maximum voltage allowed for the system? How many modules

we can connect in series?

NEC 690.7 specifies that our worst-case voltage, the highest voltages that the

DC array can create, must fall within the limits of the inverter.

The exact definition states that: The Voc of each module times the number of

modules in a string, correct for lowest expected ambient temperature in the

arrays location.

For the PVI 4000, maximum acceptable voltage is 600 VDC.

To calculate the maximum number of modules allowed, we need a few pieces of

data

Voc at STC for the module at 77F/25C = 50.3 volts The temperature coefficient for the module. Typically given in volts per degree C

or % voltage per degree C. You will find all this data on any module spec sheet =

-.14V/C

The lowest and highest temperatures seen in the specific jurisdiction. Below isthe data for Houston from weather.com = 9F or -13C


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Here are the calculations for the max system string size. The goal in determining

the maximum system voltage is to make sure that power production from the

array will never kill the inverter.

1. Temperature coefficient. -13C lowest temperature 25C STC = -38C changefrom STC

2. -38C X -.14V/C = 5.32 voltage increase. (negative times a negative is a positive)3. 50.3 volts + 5.32 = 55.62 is the highest voltage we will ever expect to see from

each module, and this is the voltage we will use to determine the maximum

number of modules in a string.

4. 600VDC (highest acceptable inverter voltage) / 55.62 = 10.78 modules.5.

We round down to 10 modules, because we cannot go over 600 volts.

6. Maximum system voltage (MSV) = 10 modules X 55.62 = 556 voltsHow do we calculate the minimum number of modules in a string?

The goal of calculating the minimum number of modules in a string is to make

sure that in the worst case scenario, when the array is extremely hot, the system

will still produce enough voltage to turn the inverter ON. Thus, were looking to

understand the lowest possible voltage the system will create.

Heres what we need:

1. Vmp of the module. Operating voltage of the module under load = 40.7 volts2. Temperature coefficient correction factor for the module from STC = -.14V/C3. The highest temperature recorded for the location youre installing = 106 degrees4. The bottom range of acceptable voltage for the MPPT range for the inverter =

200 V DC

5. Ambient air correction factors from the conduit that the electric wire will be in.This can be looked up at NEC Table 310.15 B2C. Based on how far the conduit

is off the roof, it will give us the temperature that we need to ADD to the highest


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temperature to derate module performance. Why? wires heat up more when

sitting in conduit rather then outside air.

Here are the calculations.

1. The conduit will be placed 2 inches above the roof. Thus, we must add 40F to theoutput temperature of 106 to find the temperature we will derate the modules by

40 + 106 = 146. 146F = 81C

2.

81C 25C (STC) = 56C above STC. Remember, voltage is indirectly related totemperature. Higher temperature equals lower voltage. Thus, the hottest

conditions the array will ever see is 56C higher then the STC voltage.

3. 56C X -.14V/C = 7.85 DECREASE in voltage per module4. 40.7Vmp 7.85v = 32.84 Vmp (at 149F) What this means is that is the array is

under load (being used) and its the hottest that its ever been in Houston (109F)

and the conduit is 2 inches above the roof, we can expect that each module will

be producing 32.84 volts.

5. 200VDC (the minimum volts needed to turn on a PVI 4000) / 32.84Vmp = 6.09modules. For this we need to ROUND UP (if we go down the inverter won t turn

on) so our conclusion will be 7 modules.

Conclusion on Voltages

With a Sanyo 205 module, we can have between 7 and 10 modules given the

voltage ranges of a PVI 4000 inverter.

However, now we need to make a table to figure out how many strings to haveand the proper number of strings to produce enough POWER (watts) for the

inverter.


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We could select either 2 strings of 10 modules or 3 strings of 7 because both will

produce enough DC power to power our inverter.

We will select 2 strings of 10 modules for two reasons.

Our roof only has enough space for 20 modules so 21 will not fit on it.

All else equal, its better to have fewer strings and more modules per stringbecause higher voltages = less voltage drop because less amperage will be

flowing for the same amount of power.

The conclusion from Solectrias string inverter tool match our findings done by

hand


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String Sizing Tools are Readily Available and Free

All of these calculations are typically done with software or with an invertermanufacturers string sizing tool. Here are three free options:

Solectria String Sizer Advanced Energy String Sizer Fronius

However, its good to understand the theory behind their calculations.

4. How do we size conductors?


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After weve selected the size of the array and the inverter we need to size the

conductors that will be used.

The purpose of conductor sizing is to make sure conductors can hold

the amperage that they need to help. The ampacity rating of a conductor is the

current it can safely conduct without overheating. The reason conductors cannot

overheat is because the insulation on the outside will melt and the faults will be

more likely to occur.

Current causes heat in conductor due to resistance of the wire Bigger wires = lower resistance

Lower resistance = less heat. Too much heat = insulation melting = faults, arcs, death and fire. Insulation rating determine ampacity

Most people, including myself, find this extremely confusing at first. So before we

start talking about conductor sizing, lets take a look at the problem from 30,000

feet to understand logically what is happening:

1. Understand how much the conductor NEEDS TO CARRY. The first thing weneed to understand is how many amps need to flow through a section of wire.

When looking at solar PV project they come into two main group, solar PV source

circuits (those from after the modules and before the inverter) and non-solar PV

source circuits (those coming after inverter)

2. Understand how much the conductor CAN CARRY based on its rated ampacityAND conditions of use. Well talk about how to adjust a wire based on conditions

of use and its rated capacity.

3. Thus, for every conductor sizing example, we should always be asking ourselves,HOW MUCH does the conductor NEED TO CARRY and HOW MUCH can theconductor carry. In all cases, the conductor MUST be able to carry more then it

must carry.


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With that in mind, there are four areas that we need to consider when sizing

conductors.

1. Standard ampactiy tables based on a) continuous or b) maximum current.2. Derating wire for conditions of use3. Fuse or OCPD protector rating4. Terminal Rating

1. Standard Ampacity Tables based on continuous or maximum current.

Determining ampacity requirements based on continuous or maximum current.

Remember this is calculating HOW MUCH a conductor needs to carry. Isc = Rated short circuit current which is the maximum current flow when the

positive and negative are connected together at STC. Our module has an Isc of

5.54A

Maximum Current. NEC 690.8A Circuits that are supplied by solar PV modules(anything before the inverter) can deliver output current that is HIGHER than their

rated short circuit currents. Rated short circuit is at 1000W/M2 irradiance. Real

conditions can see 1250 W/M2. > Thus Isc X 1.25 = Maximum solar pv source

circuit current

Continuous Current. NEC 690.8B1 and 210.19A1. Continuous loads can only beloaded to 80% of its capacity. Solar PV array output AND inverter output are

always considered to be continuous since they last for more then 3 hours. Thus,

10amps (max Isc) x 1.25 = 12.5 amp conductor.

To understand which needs to be applied to what circuits, its easiest

to separate between solar PV circuits (before the inverter) and non-solar PV

circuits (after the inverter)

Solar Generated Circuits = Isc X 1.25 (high current) X 1.25 (continuous load) =Isc X 1.56 = the required conductor ampacity for a solar source circuit

In our example, the Isc is 5.54 amps and we have 2 strings. Thus, our conductorsmust be able to carry 5.54 X 1.56 X 2 = 17.28 amps.


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Inverter AC circuits = Rated current X 1.25 (for continuous use) = requireconductor amapacity. Note: many inverter manufactures will specify simply the

continuous AC output current, so you dont need to perform this calculation. For

the PVI4000, thats 16.3 amps.

2. Derating Wire for Conditions of Use

Now that we understand how to calculate HOW MUCH conductors need to carry,

we need to select a conductor that can carry that current in the conditions where

it will be used. All else equal, the hotter the surrounding air that conductors are

placed in, the less amperage they can safely carry and still meet our ampactiy

ratings for safety.

There are three main things to consider:

1. The rated capacity of the wire at testing conditions

2. The effects of temperature where it will be used

3. The effect of conduit fill

1. Standard Conductor Ampacity

The NEC has tables of ampacity for different conductors depending on size and

the insulation used. The standard ambient temperature is assumed to be 30C.

Table 310.16 is for conductors in conduit or earth Table 310.17 is for conductors in free air.


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Below is a sampling from 310.16

Example: From our example, what is the smallest size wire that can be

used from a combiner box that combines 2 strings of Sanyo 205 modules

in parallel?

Min Amapacity = Isc X Number of Strings X 1.56 = 5.54 X 2 X 1.56 X =

17.28 amps

This can be satisfied with by a AWG 14 THWN-2 Conductor

Why? We NEED to carry 17.28 amps. AWG14 CAN CARRY 25 amps. 25

> 17.28

2. Adjusted for Conduit Fill


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If there are more than 3 current carrying conductors in a raceway or cable, the

conductor ampacity must be derated for conditions of use per NEC 100, this

excludes grounding conductors per NEC 310.15B5

The impact of conduit fill is essentially the same as the increase in temperature,

more conductor in a conduit, and where that conduit is, will impact how hot it gets

in that conduit and thus how much the conductor can carry.

Table 310.15B2a gives factors for derating. Some value from 310.15 are below.

From our example, we only have 2 strings ( 4 home runs) from the array.

Lets assume that we have 40 modules (10 modules per string, 4 strings),

that we are combining, so we have to combine 2 arrays. 8 source circuits

into 4.

What would be the minimum amapcity in that situation?

Number of conductors = 4Min ampacity needed for two conductors = 17.28

Adjusted ampacity for conduit fill = 17.28 / .80 = 21.6

14AWG still works because it has a maximum capacity of 25 amps.

3. Adjusting for Temperature Rating

The conductors are tested and their ampacity is rated for 30C. Thus, if the

conductors will be used in any condition that may be higher then 30C, we need to

reduce their ampacity ratings. NEC provides table to perform these calculations

in table 310-16 and 310-17.


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Below is a sample from 310-16.

Example. Our system will be installed in Houston, TX, the highest

temperature is 106F. Can we still use 14AWG to combine our source

circuits?

14AWG Standard Amapacity = 25 amps.

Adjusted Ampacity = 25 amps X .87 = 21.75 amps.

This means that given it 106 outside, 14AWG is rated to carry 21.75

amps. We need to carry 17.28 amps. So 14AWG still works.

The temperature derating also needs to be adjusted based on the distance that

the conduit is above the roof.

Per NEC 310.15B2C, the below determines the temperature that needs to be

ADDED to the highest ambient temperature based on where the conduit is

placed.


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From our above example, if the conduit was being run 1 inch above theroof, we would need to ad 40F to the ambient temperature of 106F to

equal 146F.

This means that a 14AWG with a rated capacity of 25 amps will have an

ampacity of 25amps X .58 = 14.5 amps.

Lets walk through a full example to make sure we have all these concepts in

order.

What we need to make sure is that corrected amapacity of the conductors (the

rated capacity derated for use) is GREATER then the maximum amount of

current that will be flowing through the conductor.

We have 2 strings of 10, 205 watt modules

Minimum Ampacity = 2 parallel strings X Isc X 1.59 5.54 X 2 X 1.56 X = 17.28 amps 17.28 amps is REQUIRED

We have 2 conductor pairs, 4 home runs running to the DC disconnect and

interview. Conduit filled is .80

The highest ambient temperature is 109F/43C


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The homeruns will be in conduit that is 1/2 inches above the roof. This adds 33C

to the 43C ambient temperature equaling. Our temperature derating is now 76C

or .41

Thus, the equation is Conductor Ampacity X conduit fill derate X ambient

temperature derate.

14AWG is rated for 25 amps. 25 amps X .80 X .41 = 8.2 amps. Under these conditions 14AWG is only rated to carry 8.2 amps. WE MUST

CARRY 17.28 amps.

We would increase our conductor size to #8 AWG

8AWG 55 amps X .80 X .41 = 18.04 amps. 18.04 conductor ampacity > 17.28 minimum ampacity needed.


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AbouttheAuthor

Chris Williams is the Chief Marketing Officer at HeatSpringMagazine.

He writes at Cleantechies, Alternative Energy Stocks andRenewable Energy World. He's a clean energy jack-of-alltrades. He has installed over 300kW of solar PV systems,

tens of residential and commercial solar hot water systemsand 50 tons of geothermal equipment. Chris is an IGSPHA

Certified Geothermal Installer and will be sitting for hisNABCEP in September.

If you have any questions.If you read this guide and have any questions or want to get more article,whitepapers or information there are a few ways to keep in touch.

Subscribe to HeatSpring Magazine with your RSS subscribers.

Ask HeatSpring a question on our facebook page.

Ask me, Chris Williams, @topherwilliams, a question on twitter.

Subscribe to HeatSpring TV, our video podcast toget updates on interviews with industry experts on best practices.

Join our linkedin group to connect with HeatSpring alumni andother professionals.Email. You can email directly: [email protected]. If you have an in-depth question call me at 617 702 2676


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