Solar Panel Output Calculator — Daily, Monthly & Annual kWh
Calculate how much electricity your solar panels will produce per day, month and year. Enter system size and peak sun hours to get accurate kWh production estimates.
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How to use this calculator
- Enter your solar system size in kilowatts (kW). A typical US home system is 6–10 kW.
- Set your daily peak sun hours using the state selector on the right, or enter a custom value. US average is 4.0–5.5 hrs/day.
- Adjust the system efficiency factor (default 80%) — this accounts for inverter losses, wiring losses, shading and temperature.
- Set annual degradation rate (default 0.5%/yr) — this is how much output drops each year due to cell aging.
- Daily, monthly, annual and year-25 production estimates appear instantly.
Understanding your results
How solar output is calculated: Daily output (kWh) = system size (kW) × peak sun hours × efficiency factor. A 7kW system with 5 peak sun hours and 80% efficiency produces 7 × 5 × 0.80 = 28 kWh per day.
Peak sun hours vs hours of daylight: Peak sun hours are not the same as hours of daylight. Peak sun hours measure the total solar energy received in a day, expressed as equivalent hours of full-intensity sunshine (1,000 W/m²). Los Angeles has about 5.8 peak sun hours per day; Seattle about 3.8; Phoenix about 6.5. Use the state selector to load your local value.
System efficiency factor (derate): Real-world solar systems never achieve 100% of their theoretical output due to: inverter conversion losses (~2%), wiring losses (~2%), soiling/shading (~3–5%), temperature effects (~3–5%) and clipping losses (~1–3%). The standard NREL derate factor is 0.86 (86%), but for conservative estimates 0.80 (80%) accounts for real-world conditions including partial shading.
Annual degradation: Solar panels degrade about 0.3–0.7% per year. At 0.5%/yr, a system producing 10,000 kWh in year 1 produces 9,500 kWh in year 10 and 8,750 kWh in year 25. Most manufacturers guarantee 80% of rated output at 25 years (implying maximum 0.8%/yr degradation).
Seasonal variation: Solar output varies significantly by season. A system producing 30 kWh/day in June may only produce 12 kWh/day in December in northern states. Annual totals are more meaningful than single-day estimates for financial planning.