The size of your solar electric system depends on how much electricity you use. For maximum payback, you should generate enough electricity over the course of a year to cover the amount that you use in a year.
Generally that means looking at your annual electric bill and sizing a system to generate that much electricity. The electricity you use is measured in megaWatt hours. The example we use assumes 6 megaWatt hours ( 6 mWh) of consumption per year.
A typical conversion factor from system size, which is measured in kiloWatts (kW), to annual usage in mWh is 1200. That is a 5 kilowatt system will produce about 6 megawatt hours of electricity per year. The exact conversion factor depends on orientation, the modules you pick, and how much sunshine there is in any given year.
The installer will look at your electric bill and make a recommendation to you.