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.

Sunify will eliminate paper notes so you no longer have to copy and paste notes into emails and waste time.
Sunify will ensure that you, or the sales people that you manage, capture the information that they need to on the first visit.
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.
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.


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:

550 kWh/month / 30 days per month = 18.33 kWh per day
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
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
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.