Solar Panel Calculator

System size comes from consumption, not roof size. Take your monthly kWh from the utility bill, convert to a daily average, then divide by peak sun hours times a system efficiency factor: kW = daily kWh ÷ (sun hours × efficiency). Peak sun hours is not daylight hours — it's the equivalent number of hours at full 1,000 W/m² intensity, typically 3 to 6 depending on where you live.

That's 29.6 kWh a day. At 4.5 peak sun hours and 20% system losses, the usable yield is 3.6 kWh per kW per day, so you need 8.22 kW. With 400 W panels that's 21 panels — an 8.4 kW installed system after rounding up, producing roughly 11,000 kWh a year and covering about 102% of your usage. Expect around 440 sq ft of panel area, before any roof setbacks.

Nameplate wattage is measured in a lab at 25°C. In the real world you lose roughly 2–3% in wiring and connections, 2–4% in the inverter, 2–3% to soiling, and the big one — 5–15% to heat, since panels lose about 0.3–0.5% of output per degree above 25°C. Add panel mismatch and about 0.5% degradation a year. Fourteen to twenty-five percent total is normal; only a pristine, cool, unshaded site reaches the low end.

In the northern hemisphere, a south-facing array at roughly your latitude's tilt is optimal; east or west facing costs 15–20% of annual output, and north is usually not worth doing. Shade is harsher still, and non-linear — one shaded panel can drag down a whole string unless the system uses microinverters or optimizers. A few hours of afternoon shade from one tree can undo the benefit of several extra panels.

Whether to cover all your usage depends on how your utility credits exports. Under full retail net metering, overbuilding slightly makes sense because surplus banks at full value. Where exports are credited below retail — increasingly common — a smaller system sized to what you consume during daylight pays back faster. Check your utility's rules, and factor in an electric vehicle, heat pump, or battery before fixing the size.