With our solar yield calculator, you can easily estimate how much energy you could generate with a photovoltaic system on your roof. Enter your roof area, orientation, tilt angle, and location – we'll calculate your expected yield. can generate around 21,840 kilowatt-hours (kWh) of solar electricity annually—that's more than most homes need. But also, the world isn't perfect. Realistically, your roof's solar. . Here you basically have to input the total roof size, and the calculator will tell you how many 100-watt, 300-watt, or 400-watt solar panels you can put on your roof (theoretical maximum). Number Of Solar Panel By Roof Size Chart. Rooftop potential is not equivalent to the economic or market potential for rooftop solar—it doesn't consider availability or cost. Formula: Panels = (Roof Area × Usable % × (1 − Spacing Loss %)) ÷ Panel Area → Total Capacity (kW) = Panels × Panel Wattage ÷ 1000. It also factors in subsidies, making solar energy. .