Part 2: How to Design PV – A Walkthrough of How to Size a Solar Array and Estimate Power Production

This is the 2nd article in a series about how to design solar PV projects. We started with solar 101, the basics. If you’re 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. If you’re looking to start a solar business but are brushing up on the technical side, read our solar startup guide article to get more free guidance on solar sales and finance.

Download the free full PDF “Solar PV Design and Installation 101” guide here

Also, this post is specifically focused on basic technical understanding of solar PV projects. However, more and more we’re getting questions from contractors that need to understand how to finance commercial solar projects.

• Click here to sign up for our Solar MBA and Learn how to Finance Commercial Solar PPAs from A to Z. Click here to test drive the Solar MBA for free. 
• Click here to join our Linkedin group “Best Practices for Financing Commercial PPAs Between 200kW and 5MW” and continue the conversation about best practices.
• Listen to 60 Minute Interview: Advice from a $20MM Solar Tax Equity Investor to Commercial Solar Installers – Focus on a Niche, Be Fast, and Standardize your Operations
• How to Finance Non-Profit Solar Projects – 50 Minute Session Answering 5  Key Questions
• 60 Minutes of Video Answer 7 Questions on Best Practices for Setting up Commercial Solar Power Purchase Agreements.

If you’re brand new to solar, I also get a lot questions about the NABCEP Certification.  Click here to learn what is NABCEP and wether or not you should need to get the certification. If you’re serious about the solar industry and you want to get the NABCEP Certification, but you need to understand how exactly to apply, you can read more about getting the NABCEP Certification here.

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 I’ll 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 you’ll 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 we’ll 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, I’d argue that it’s 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.

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 we’ll 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 Considerations

3. 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 day

4. 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.

You Need to have standard process to collect all of this information. Performing high quality and efficient site visits is absolutely critical to the success of profitable solar projects, especially residential projects! You need to be able to capture all of the information you need to 1) quote the system correctly 2) design the project and 3) inform the installation crew what to expect. An efficient site visit process will lead to smooth operations and profitable jobs while complex process can lead to unprofitable jobs and a lot of confusion.

Click here to check out Sunify. Sunify is a simple mobile tool that solar sales people use to make sure they collect all the information they need on a site visit with the least possible effort. It’s so cheap it will pay for itself in one site visit. Sunify does 4 things that will make your site visits better.

1. Sunify will eliminate paper notes so you no longer have to copy and paste notes into emails and waste time.
2. Sunify will ensure that you, or the sales people that you manage, capture the information that they need to on the first visit.
3. You’ll collect better quality information because you can collect video and audio notes in addition to photos and text answers. This will give lead to more accurate quotes, design, and an easier time for the installation team.
4. It’s all the tools you need in one place, so you’ll never loose your notes again.

Click here to check out Sunify. 

1. Customer Contraints. 

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 client’s annual energy usage. However, because most property owners use so much power, and the power density of solar PV is so low, it’s rare to have an array that can produce 100% of the power with solar power. It’s typical that the solar fraction of a project (total power used / power supplied by solar) is less then 30%.

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 the solar PV financing article because it’s critical to understand the value of the power that a solar project produces. Right now, we’re just concerned with pure design.

If you need to learn more about what demand charges are, I’ve 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

Let’s 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, here’s how:

1. 550 kWh/month / 30 days per month = 18.33 kWh per day
2. 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 AC needed 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 DC

If 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.

We’ll 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 it’s a cash or lease sale. Learn more about lease sales in our free course about solar lease.
• If it’s a cash customer, make sure you understand what their budget is. Make sure you understand if they are purchasing cash, or with a home equity line of credit or wrapped into a mortgage for new construction.

2. Site Contraints

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 it’s 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: I’m 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 doesn’t 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 it’s good practice 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. Common examples include: chimneys, stink pipes, eaves, shading from another part of the roof.
• A good rule of thumb for local shading is don’t place modules anywhere that is closer then 3x the height of the obstacle from the object. If a stink pipe is 12 inches, don’t 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 shading elements. 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 also be from buildings. It is shading that occurs off the roof that impacts the amount of irradiance hitting the roof.
• It’s best to have no shading between the hours of 9am and 3pm for the whole year. 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 you will 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 you’ve lost due to shading. If it’s 95%, you’ve lose 5% production from the best case scenario due to shading.
• Key to remember: Trees Grow. If you’re building an area that has some shading, when you perform your power production estimates it will be good to assume your shading will increase by a small amount each year, let’s say .5%.
• Key to remember: Some states have rebate programs that say a roof must solar access of at least 80%.

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

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

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 have a 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 footprint AND HEIGHT of the AC units, skylights, height of knee walls, and all other equipment.
• Examples below of skylights, knee-walls, AC units, and existing conduit.

• 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.

Example: Houston, Texas House

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

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

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.

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.

5. Determine how many modules can fit in the adjusted usable space based on the size of the module and racking. You’ll need 3 things

1. The amount of usable space on your roof
2. The dimensions of your module
3. Needed space for racking

A few other key tips to keep in mind.

•  It’s good to make sure the modules do not overhang the ridge. It’s good for the space 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, but sometimes 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 we’re going to use:

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)

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 it’s 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.