You’ll need to know three key factors to calculate how many solar panels you need:
- Your annual electricity usage
- Your solar power system’s estimated production ratio
- The wattage of the solar panels you’re considering
You’ll then divide your annual electricity usage by the estimated production ratio, and divide that number by your solar panels’ wattage. We’ll break down these factors and the formula in more detail below.
Annual Electricity Usage
Your energy consumption is the amount of electricity your home uses annually. This number is measured in kilowatt-hours (kWh) and includes all lights and appliances, such as air conditioning units, water heaters, and refrigerators. The U.S. Energy Information Administration (EIA) reports that a U.S. residential customer’s average annual electricity consumption was 10,632 kilowatt-hours (kWh) in 2021, with an average of 886 kWh per month.
A solar panel system’s production ratio is its estimated energy output over time (in kWh) compared to its actual system size (in W). You might assume this is a simple 1:1 ratio, but complex variables come into play, such as the amount of sunlight your home receives versus the amount of shade.
For example, a 10-kW system that produces 14 kWh of electricity in a year has a production ratio of 1.4 (14/10 = 1.4). This applies to a home in a sunny state such as Arizona, where there are large amounts of sunlight all day, year-round. If your home is located somewhere like New York, where it can be cloudier and there’s more rain, this production ratio might dip to 1.2.
This is a complicated ratio, so we advise you to contact a professional for the best results.
Solar Panel Wattage
You might think that the best solar panels are all built the same, but that’s not true. There are different solar panels for different projects with different energy outputs. A panel’s wattage is how much electricity it emits, and most residential solar panels range between 300 and 450 watts of power. The higher the wattage, the fewer panels you’ll need.
The actual formula a solar installation company will use to quote you a solar panel system is as follows:
- Number of panels = system size/production ratio/panel wattage
If we use a simplified version of the numbers we listed above, we get the following:
- Number of panels = 11,000 kW/1.4/300
This formula equals approximately 25–27 panels. However, you may require fewer or more solar panels depending on your specific home. The National Renewable Energy Laboratory (NREL) maintains a calculator to help you estimate your needed system size.
Other Ways To Calculate How Many Solar Panels You Need
A simple alternative to the formula above is to look at your monthly electric bills and determine how much energy you use. Then, multiply your energy usage by the hours of sunlight your home gets, and divide that result by the wattage of the panels you’re considering.