How the solar system size calculator works
From bill to kilowatts
The chain is short: bill ÷ rate = monthly kWh; ÷ 30 = daily need; ÷ (local sun-hours × derate) = system kW. Each step is arithmetic you can audit — which is the point. A quote that jumps from your bill to a system size without showing this chain is asking you to skip the only part that protects you.
Sun-hours are geography
"Peak sun-hours" are equivalent full-sun hours per day: ~3.5 in the cloudiest corners of the Northeast and Pacific NW, 5.5+ across the Sun Belt, 6.5+ in the desert. The same 90% offset needs ~40% more panels in Seattle than in Phoenix — which is why national "average" system sizes are nearly meaningless.
Oversizing is a thing too
Utilities compensate excess production differently — full retail credit (net metering), reduced rates, or hourly credits. In many post-net-metering markets, the optimal offset is 90–110% of usage, not 150%. Bigger is not better; right-sized is better. The offset input exists so you can test both directions.