How To‎ > ‎

Evaluate Your Project

Determine your current electricity usage

Using your electric bills, find your total electric usage (preferably for at least the last 12 months) in kilowatt hours (kwh).

  • If you are an Eversource customer, you can download what you need by creating an account at www.eversource.com and going to View Account > Usage History.

This will help determine what size system is best for you. You may choose to target replacing just part of what you use now, all of it, or even more if you anticipate increased usage.

Determine how much sun you get.

Rooftop systems are generally the most cost effective because you use existing structural support, have shorter wire lengths, and may have less shade because your panels are higher, but there are alternatives.

If your home is built to modern building code standards you should not need any structural changes. See this NH Structural checksheet for rooftop PV.
You may have a better location than your roof; some installations are on the ground or on a pole.

Facing due South is optimal, but small differences (20-30 degrees) make very almost no difference and greater differences can still work well; panels that face even due east or west can generate a lot of power.

  • Use this tool to determine the Azimuth (what compass direction your site faces) of your roof or other possible install location, or you can use the Compass in most smartphones to get a good enough measurement (make sure you set the compass to show “true” directions rather than “magnetic”; in our area, the difference is a little over 14 degrees).

You will need to know the Pitch angle of your installation. 

  • If you are mounting on your roof, use this Roof Pitch calculator to find the pitch of panels mounted flush to it. 
  • If you will mount on the ground, you can choose your angle (approximately 38 degrees is optimal in our area; small differences either way do not make a big difference).

Once you have your Azimuth and a Pitch angle, 

  • Use this estimator to show how the azimuth and pitch affect the amount of sun your location will get through the year (Annual Insolation). This accounts for average weather, but not for any local shading of your site.

Determine how much shade your site gets through the year. There are smartphone apps that can do a good enough approximation for you:

Calculate how much power your system would produce

  • The National Renewable Energy Laboratory (NREL) provides this excellent calculator to predict the energy output from a system using the measurements above and different panel configurations. Recent HAREI member projects have used:
    PVWatts Calculator

    • panels whose maximum DC output is 290W
    • micro-inverters whose maximum output is 250W AC (solar panel output decreases a little over time; by using panels with slightly higher output, the net output after conversion does not decrease).
    • In the calculator, most default parameters other than the location specific items above are good enough.


Having determined the goals and parameters for your system, you can go on to turn that into a system design.