Solar Panel Output Calculator
Enter panel wattage and your location's peak sun hours to get realistic daily, monthly and yearly energy — including the standard loss derate.
Last updated: 2026-09-15
How the calculation works
- Panels are rated at Standard Test Conditions (1000 W/m², 25 °C). PSH converts your location's daily irradiance into equivalent full-sun hours.
- Multiplying Wp × PSH gives the ideal energy; the derate factor then subtracts real losses (inverter, wiring, soiling, heat).
- Daily energy is extrapolated to monthly and yearly averages.
Formula
E = Wp × PSH × derate
| Symbol | Meaning | Unit |
|---|---|---|
E | Daily energy | Wh |
Wp | Peak wattage (STC) | W |
PSH | Peak sun hours | h/day |
Worked example
Interpreting the result
Real output varies seasonally — a 4.5 PSH annual average might mean 6 PSH in summer and 2.5 in winter at mid-latitudes. Size systems for the worst month if you need year-round reliability. The derate factor dominates long-term yield: dirty panels in dusty climates can fall well below 0.8 without cleaning.
Assumptions
- Fixed tilt approximately equal to latitude, south-facing (northern hemisphere).
- Panel output at STC; real nameplate tolerance ±3%.
Limitations
- Does not model shading from trees, chimneys or adjacent rows — partial shading can halve output.
- Temperature losses vary widely by climate and mounting airflow.
- Grid-tied systems with clipping (DC/AC ratio > 1.2) behave differently.
Frequently asked questions
How much does a 400 W panel produce per day?
Depends on your PSH: at 4.5 PSH with typical losses, about 1.4–1.5 kWh/day. Multiply by 365 for the yearly figure (~525 kWh).
What is a good derate factor?
0.75–0.85 is realistic for a well-installed fixed system. Optimized string inverters with clean panels can reach 0.85+; dusty or hot rooftops can fall to 0.7.