How Do I Calculate How Many Solar Panels I Need?

One of the most important steps in planning a solar project is to determine how many panels you will need to power your home. The number of panels is the key factor in figuring out how expensive it will be to outfit your home to use solar power. This calculation will require you to have an electric bill for December that includes a summary of your electricity usage for the other months of the year, or all of your bills for the year.

A solar-panel installation

Step 1

Calculate your average electricity use per day in watts using your electric bill for December. Your electric bill should have your usage for each month on the bill, add the total kilowatt hours you used for the year and divide by 365 to get your average electricity use. Multiply that result by a thousand to get the result in watts. Your electric company may already provide this number on the bill.

Step 2

For example, say that you used your electricity usage for each of the 12 months was 340, 325, 330, 345, 340, 320, 315, 325, 330, 335, 330 and 345. Adding these numbers together yields 3,980 kWh. Then divide this figure by 365 days in a year: 3,980 / 365 = 10.904 kWh per day. Multiply by 1,000 to convert this to watts: 10.904 * 1,000 = 10,904 watts per day.

Step 3

Find out the average hours of sunlight per day in your area. To do this use an online solar calculator, such as the one in Resources. You will only need to know your location.

Step 4

Divide the number of watts of power you use each day by the average hours of sunlight per day. This will be the number of watts you need to produce per hour. Continuing the example, suppose that you get about three full hours of sun per day. 10,904 watts / 3 hours = 3,635 watts / hour.

Step 5

Divide the watts you need to produce per hour by the rating of the panels you want to purchase and round up. This will tell you how many panels to use. Different panels will have different watt ratings, so you have choices about which ones to purchase. For the example, if 500-watt panels were being purchased, you would need 3,635 / 500 = 8 panels.