Solar Panel Output Calculator
Enter panel wattage and your location's peak sun hours to get realistic daily, monthly and yearly energy (kWh) — including the standard loss derate.
Open calculatorLast 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.
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.
Peak sun hours are the number people get wrong
Peak sun hours are not daylight hours — they are the equivalent hours of full 1,000 W/m² irradiance per day. A location with 12 hours of summer daylight might deliver 6–7 peak sun hours; in winter the same spot can drop to 2–3. Sizing a system on an annual average hides that seasonal swing: an off-grid system that works in July fails in December. Use the worst-month figure for off-grid sizing, and the annual average only for grid-tied savings estimates.
Real-world derates the textbook formula skips
- Temperature: panel output falls roughly 0.3–0.5% per °C above 25 °C cell temperature — a hot roof can cost 10% of nameplate on a summer afternoon.
- Soiling: dust, pollen and bird droppings cost 2–5% typically, more in dry climates without rain to clean the array.
- Orientation and tilt: the correction factor this calculator applies assumes reasonable tilt; flat-mounted panels lose 10–15% versus optimal tilt at most latitudes.
- Inverter efficiency: grid-tied systems lose a further 3–6% converting DC to AC — the panel figure here is DC-side.
Reverse sizing: How many solar panels for your monthly electric bill (kWh)?
If your electricity bill lists total monthly consumption (e.g., 900 kWh/month) and you want to calculate how many solar panels you need, use this practical reverse sizing method:
1. Calculate daily consumption target: Monthly kWh ÷ 30 days = Daily target (e.g., 900 kWh ÷ 30 = 30 kWh/day).
2. Calculate daily energy per 400 W panel: 400 W × 4.5 Peak Sun Hours × 0.80 Derate ÷ 1,000 = ~1.44 kWh/day per panel.
3. Divide daily target by single panel output: 30 kWh/day ÷ 1.44 kWh/panel ≈ 20.8 → 21 panels needed (an 8.4 kW DC system).
Adding a 10%–15% margin covers cloudy winter days, seasonal sun angle shifts, and normal module degradation over time.
Frequently asked questions
How do I calculate kWh from solar panel wattage?
Multiply panel wattage (W) by daily peak sun hours (PSH), multiply by the derate efficiency factor (typically 0.80), and divide by 1,000. Formula: Daily kWh = (Watts × PSH × Derate) / 1,000.
How much does a 400 W panel produce per day?
Depends on your PSH: at 4.5 PSH with typical losses (0.8 derate), about 1.44 kWh/day. Multiply by 365 for the yearly figure (~525 kWh).
How many solar panels are needed for 30 kWh per day?
At 4.5 peak sun hours with standard 400 W panels (producing ~1.44 kWh/day each), you need approximately 21 panels (around 8.4 kW total system size) to meet 30 kWh daily demand.
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.