How to Determine Your Solar Energy System Size

To begin, calculate your average energy usage in kWh. Collect your electric bills for the past 12 months or more.

Get an average of kWh used on a monthly basis. In the image above, their monthly usage for one month was 1,825 kWh. Let’s say that, for you, this number is 1,100 kWh per month.

Now divide your average monthly usage by 30 to get your average daily kWh usage. In this case, it would be about 37kWh per day.

To keep things simple for this rough estimate, we’ll assume that a 250W solar panel will produce about 1kWh a day. **Keep in mind that this is a rough estimate, based on a site location getting 4 hours of sunlight per day**

Assuming that you use about 37 kWh per day, you would need about 37 solar panels to meet 100% of your average energy needs.

Based on your current usage, how much of this do you want to offset with a solar power system? 100% …75%…. 50%? Multiply accordingly.

Let’s say you wanted your solar system to account for 80% of your energy consumption.

37 kWh per day x 0.80 = 29.6 kWh

Since one solar panel produces about 1 kWh per day, you would use 30 solar panels to account for 80% of your average consumption.

The purpose of this is to give a rough idea of how many solar panels you will need. This sizing tool provides a quick number to go off of as you’re shopping around- it’s not a replacement for having an electrical contractor design a system for you.

Once you’ve calculated your average kWh use and determined the percentage that you plan to offset with solar, get a free solar quote from We’ll help you design a complete system that’s specific to your roof and geographical location.

Photo credit: David Lee