How many solar panels do I need?

Work out system size in kilowatts, the number of panels, and roughly how much roof they'll take.

—System size

—Panels

—Roof area (approx.)

—Yearly usage covered

System kW = yearly kWh × offset ÷ (sun hours × 365 × (1 − losses))

Example: 10,560 kWh a year in a 5-hour state with 22% losses needs 10,560 ÷ (5 × 365 × 0.78) = 7.4 kW, or about 18 panels at 430 W.

Each modern panel takes roughly 21 to 22 square feet. Leave space for fire-code setbacks at roof edges and ridges.