
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.
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.
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:
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:
- 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: