Why the derate factor matters
A 1 kW panel array does not deliver 1 kW. Inverter losses, high panel temperatures, wiring resistance, dust and imperfect angles combine to cost roughly 20–25% of nameplate capacity. This calculator applies a 0.78 factor, which is the standard industry assumption.
Peak sun hours is not daylight hours. It is the equivalent number of hours at full 1,000 W/m² intensity — a twelve-hour day might deliver only five peak sun hours.
Payback depends on policy as much as physics
The simple payback above assumes every generated unit displaces a purchased one. In practice that depends on your net-metering rules: full net metering makes the assumption roughly true, while low export tariffs mean daytime surplus is worth far less than what you pay at night.
Subsidies, tax credits and rising electricity prices all shorten payback; battery storage lengthens it substantially while improving resilience.
Frequently asked questions
Should I size for my full usage? +
Not always. If your export tariff is poor, sizing to your daytime consumption rather than your total often pays back faster than a larger system.