Annual Energy Consumption
The first step in the calculation is to determine your annual electricity usage — the amount of electricity your home consumes in a year. This is measured in kilowatt-hours (kWh) and should include every energy source in your home, such as appliances, lights, air conditioning units, and water heaters. According to the U.S. Energy Information Administration, the average American household uses 11,000 kWh of electricity annually.
Panel Wattage
The next step is determining the wattage of the panels you want to install. A panel’s wattage is the amount of electricity it emits. Solar panels typically range between 250 to 400 watts, so it’s safe to assume that the average panel wattage you’ll find is 300.
Estimated Production Ratios
The production ratio is a panel's estimated power output over time (in kWh) to its size (in W). This is rarely a 1:1 ratio. The amount of sunlight your panels get will vary, and your production ratio will change in response.
For example, a system that’s 10 kW and produces 15 kWh of electricity per year has a production ratio of 1.5. In the U.S., production ratios are typically between 1.3 and 1.6.
Solar Panel Calculation Formula
Once you know your annual energy consumption, panel wattage, and production ratios, you can use the formula to calculate how many solar panels it takes to power a house. The formula is
Number of panels = System size/production ratio/panel wattage
So, if we use the numbers we mentioned above, the equation would be
Number of panels = 11,000 kW/1.5/300 W
The answer is 24.4, which means we would need 24 to 25 panels to power this home.
