Solar Panels

How Many Solar Panels Do I Need?

Are you considering purchasing solar panels, but would like to estimate how many to buy?  There are a variety of factors that come into play when deciding the number of solar panels you’ll need.  The basic formula is:

kWh per month / (avg sunlight per day * 30) = kW solar system   (Coastal Solar, 2017). 

When using this formula, there are two important variables you need to know: 1) your kilowatt hours per month, and 2) the average amount of sunlight per day in hours for your region.  However, there other factors to consider beyond these two variables. 

Kilowatt Hours Per Month

This variable is easily found if you can obtain a yearly power usage summary from your power company.  Find your average from all months.  If your average isn’t stated, you can find it by adding up all your monthly usage in kWh and dividing by twelve. 

Average Sunlight Per Day

Look up average sunlight hours for your region.  Make sure the number you find represents average sunlight per day.  If it is sunlight per month, this number can still be used, but don’t multiply it by 30 as shown in the formula. 

Other Variables to Consider

After plugging the above two variables into your formula and finding the basic size of your system, there are other factors to think about.  Solar systems last an average of twenty years.  Do you plan on increasing your power usage substantially in that time?  Home additions, electric cars, and powered garages are examples of a significant increase in usage.  Do you want all of your power to come from solar, or just a portion?  Does your power company use net metering, or buy excess power you may generate?  Do you plan on going off-grid by using a battery instead?  Can your roof accommodate the necessary number of panels, or will they be installed on the ground?  These are some examples, but there may also be other factors unique to your situation. 

For professional help estimating the size of your solar system, contact us!


Coastal Solar. 2017. Solar System Equation. Retrieved from

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