Solar Panel Output Calculator
A 400W solar panel does not actually produce 400 watts all day. Real output depends on how many hours of strong sunlight you get, plus losses from heat, inverter conversion, wiring, and dirt. This calculator uses all those factors to give you a realistic estimate of how much energy your panels will actually generate and how much money that saves.
How Solar Output Is Calculated
Daily Output = Panel Wattage x Peak Sun Hours x System Efficiency
A 400W panel with 5 peak sun hours and 80% system efficiency produces: 400 x 5 x 0.80 = 1,600 Wh (1.6 kWh) per day.
What Are Peak Sun Hours?
Peak sun hours are not the same as hours of daylight. One peak sun hour equals 1,000 W/m squared of solar irradiance for one hour. A location with 8 hours of daylight might only get 4-5 peak sun hours because morning and evening sun is weaker.
| Region | Approx. Peak Sun Hours |
|---|---|
| Arizona / Southern California | 6-7 |
| Texas / Florida | 5-6 |
| Midwest / Northeast US | 4-5 |
| Pacific Northwest / UK | 3-4 |
| Germany / Canada | 3-4 |
System Efficiency Breakdown
The 75-85% efficiency factor accounts for multiple losses:
- Inverter conversion: 3-5% loss
- Wiring and connections: 1-3% loss
- Temperature derating: 5-15% loss (panels lose efficiency in heat)
- Soiling (dust/dirt): 2-5% loss
- Shading: 0-20% loss depending on surroundings
- Age degradation: 0.5% per year
Quick Sizing Guide
The average US household uses about 30 kWh per day. With 5 peak sun hours and 80% efficiency, you would need about 7,500W (19 panels at 400W each) to cover that entirely with solar.