Solar Panel Power Output Calculator — kWh per Day

This calculator estimates daily energy production from your panel’s rated wattage and your location’s peak sun hours.

How the math works

A panel’s rated wattage (e.g. 100W) is measured at standard test conditions. Real-world output is lower because of heat, dust, angle and wiring losses — a typical 0.85 derate factor accounts for this.

Daily kWh = Panel wattage × Peak sun hours × 0.85

For example: 400W panel × 4.5 peak sun hours × 0.85 = 1.53 kWh/day.

Peak sun hours

“Peak sun hours” means equivalent full-sun hours. The US Southwest gets 5–6, much of the Midwest 4–5, northern Europe 2.5–3.5. Check your local solar insolation map for accurate numbers.

Adjust for season

Output drops in winter because of shorter days and lower sun angle. If you’re sizing for year-round use, use your worst-month sun hours, not the annual average.