The math,
in the open.

We divide your daily consumption by the product of your local peak sun hours and a 78% derating factor. The 78% accounts for inverter losses, wiring, soiling, and temperature — the gap between a panel's nameplate rating and what actually reaches your wall.

The number of panels is the system size in watts, rounded up over the wattage of a single panel. We always round up so the array can meet demand even at its weakest.

Panel footprint times a spacing factor K that depends on the mounting surface: pitched roof 1.95 m² × 1.2 = 2.34 m², flat roof 1.95 m² × 1.75 = 3.41 m² (tilted rows need gaps to avoid shading), ground mount 2.58 m² × 2.5 = 6.45 m² (larger panels, wide row spacing and access paths).

The same derating factor applied across a full year. This is the energy you'd expect to produce, not the energy the panels are rated for.

Each kilowatt-hour of solar displaces roughly 0.4 kg of CO₂ that a fossil plant would have emitted — the 2023 world average for grid electricity.

Years to recover the investment, assuming each solar kilowatt-hour offsets about 15¢ of grid electricity — a blended retail-rate estimate.