Solar Panel Output Calculator

Daily energy (kWh) = system size (kW DC) × peak sun hours × (1 − losses) × orientation factor.

System size is panel wattage × number of panels ÷ 1,000, so 15 panels at 400 W is 6.0 kW.

A peak sun hour is one hour of sunlight at 1,000 W/m², the test condition panels are rated under, so an annual average of 4.5 peak sun hours means the site receives 4.5 kWh of solar energy per square meter per day.

With 18% losses a south-facing 6 kW array makes 6 × 4.5 × 0.82 = 22.1 kWh a day, about 673 kWh a month and 8,081 kWh a year.

NREL's PVWatts model, the reference tool most installers and utilities use, applies a default 14.08% system loss for soiling, shading, snow, module mismatch, wiring, connections, light-induced degradation, nameplate tolerance and downtime, and then a separate 96% inverter efficiency.

Combined, 0.86 × 0.96 leaves about 82% of the DC rating as AC output, or 18% total loss.

Use 12-15% for a clean, unshaded array with microinverters and 20-25% for dusty sites, partial shade or an older string inverter.

Panel output also drops roughly 0.3-0.4% for each degree Celsius above 25°C, which is why hot desert sites underperform their sun-hour numbers in summer.

The share-of-usage figure divides estimated monthly production by the kWh on your bill. The average U.S. home used about 855-900 kWh a month in recent EIA data, so a 6 kW array in a 4.5-sun-hour location covers about 75% of it. The dollar value assumes one-for-one retail net metering; many utilities now credit exported power at lower avoided-cost or time-of-use rates, which reduces the real savings.

Seasonal swing. These are annual averages. Monthly output in northern states can be three to four times higher in June than in December, and snow cover can stop production for days. Size a grid-tied system on annual kWh, but size any off-grid system or battery on the worst month.