First, deciding how many solar panels you need seems like simple math — you find out the total energy consumption of your household and divide it by the total output of one panel. But the potential energy output from a solar panel isn’t standard. The wattage rating of a solar panel is only one piece of the puzzle; you also have to bring the peak sunlight duration into the equation.
Multiply the daily peak sun hours for your area and multiply it with the rating to get the potential output in kilowatt-hours for the day (via Solar). Naturally, the peak sunlight hours vary significantly between states in the U.S. Some regions can get up to six peak solar hours, while others only receive three.
So a solar panel that can generate 2.7 kilowatt-hours in Arizona will only supply 1.5 kilowatt-hours in New Jersey (via Solar Reviews). To offset the fewer peak sunlight hours, you’ll need to buy more panels.
Then there’s the cost of installing the panels, which also varies between residencies and contractors. That said, U.S. residents can claim federal tax credits on the investment for buying and setting up solar systems (via Department of Energy).
Hotter climates, roof slope and direction, the landscaping around the panels (if they are ground-mounted), and nearby trees that cast shade (if they are roof-mounted) also affect the total output and, in turn, the total cost of a solar system (via Architectural Review).