Patrick Crowley

Patrick Crowley
Head of Sales & Operations

Rex Kehoe

Rex Kehoe
Director of Installers

How Many Solar Panels Do I Need?

February 16, 2019

This week’s livestream is about sizing solar systems and deciding what the best option is for you. A vague but frequently asked question in the solar industry is “How many solar panels do I need?” Let’s take a look at the various factors that help you to find the right answer. 

Factors That Determine the Size of the Solar System: 

To determine the size of your solar energy system, you have to consider these three important factors:

  • Annual electric usage
  • System’s annual production
  • Your usage and your production should cancel out

Let’s get into some more details on these factors that will help you determine the size of your solar energy system.

Electric usage

  • Annual electric usage plays a big part in knowing how to size your system.
  • The following chart shows a sample annual electric consumption bill. Depending on your utility, you will get a bill either every month or every couple of months. But almost every bill comes with a graph that shows your electric usage for the last 12 months.
  • The graph could be having an odd pattern because your electric usage would be higher during summers and lower in winters.
  • The ideal goal is to measure how much you use in a year and then select panels that can produce enough to offset your consumption.

annual electric usage

  • Basically, we should be producing enough kilowatt hours to offset your consumption.
  • It varies from month to month. Net metering carries those credits over.
  • In January, you might have to pay a little bit of the bill. However, in August, you might earn some credits. And then you can apply that to next January or whenever you need to use those credits.


  • As you can see, for a specific month, the system isn’t going to produce exactly what you need. But throughout the year, the idea is to end with net zero kilowatt hours. Ultimately, the goal is for you not to owe anything to the utility.

Factors to Decide the Number of Solar Panels You Need


Factors for sizing the systemClimate:

Where you live, how much sunlight you are getting, what is the weather like—these will play a big role in determining the size of your system. This is because the weather you live in determines your electric consumption. In Arizona where it is more hot, someone may have their air conditioner running as opposed to someone living in Massachusetts might have higher billing in winter due to electric heating.

So, climate plays a big factor not only in your consumption but also in determining your production.


No two houses are created equal. Equal consumers of energy that are neighbors might need different system sizes depending upon their roof shade and orientation. A good clean southern roof always performs better. Somebody with southwest-facing roof or even a pure south roof might need one or two panels less to achieve that same offset.


Solar panels that you are looking into also plays a role in determining the size of the system–A 300W solar panel or a very high efficient 360W solar panels can affect your system’s size. The type and the efficiency of the panels would also help in determining how many panels do you need. Higher efficiency and the wattage, the less panels you’ll need. Let’s say if you have a very tricky roof from a solar perspective. Then we will recommend higher efficiency solar panels to capitalize on the roof space.  If you have enough roof space, why pay more for just having fewer panels? You can always add two or four more panels in case of enough roof space. At the end of the day, it is all about kilowatt hours.


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