How it works
A solar panel's wattage is measured in the lab, under 1,000 W of sunlight per square metre at 25°C. Real output depends on how much sunshine your location gets, which way the panels face, shade, and losses in the wiring, inverter and from heat and dirt.
The calculator uses peak sun hours: the day's sunshine squeezed into the equivalent number of hours of full sun, averaged across the year. It multiplies the panels' watts by those hours and an 80% performance factor, then adjusts for direction and shade.
Panels facing the equator (south in the US, UK and Canada; north in Australia) produce the most. East- and west-facing roofs typically make about 15% less; a roof facing away from the sun around a third less.
The formula
kWh per day = panel watts × number of panels × peak sun hours
× performance factor (0.8) × direction × shade ÷ 1,000
kWh per year = kWh per day × 365
Worked example
One 400 W panel facing south, unshaded, with 4.5 peak sun hours (a typical US figure).
| Step | Working | Result |
|---|---|---|
| Per day | 400 W × 4.5 h × 0.8 ÷ 1,000 | 1.44 kWh |
| Per month | 1.44 × 365 ÷ 12 | 43.8 kWh |
| Per year | 1.44 × 365 | 525.6 kWh |
| Worth (US average price) | 525.6 × $0.182 | $95.66 |
| Twenty panels (8 kW) | 1.44 × 20 | 28.8 kWh a day |
The same panel in the UK, with about 2.8 sun hours a day on average, makes about 0.90 kWh a day or 327 kWh a year.
Frequently asked questions
How many kWh does a 400 W solar panel produce per day?
About 1.44 kWh with 4.5 peak sun hours, from about 1 kWh in cloudy climates to nearly 2 kWh in the desert Southwest. See how many kWh a solar panel produces per day for a full table.
How much does a solar panel produce per month?
A 400 W panel makes about 44 kWh a month on average in much of the US, but far more in summer than winter.
What are peak sun hours?
The number of hours a day that sunlight would need to shine at full strength (1,000 W per m²) to deliver the day's total solar energy. Five hours of daylight in winter might only be 1.5 peak sun hours.
Why does my system produce less than its rating?
Panels rarely reach their rated watts: heat reduces output by about 0.3% to 0.4% per °C above 25°C, and wiring, the inverter, dirt and shade each take a few percent. An 80% performance factor is typical; NREL's PVWatts uses about 14% system losses before the inverter.
Does the direction of my roof matter?
Yes. Facing the equator is best. East or west roofs make roughly 10% to 20% less but spread production over the morning or evening, which can suit time-of-use tariffs.
Sources
- NREL, PVWatts calculator (solar resource and default system losses).
- European Commission, PVGIS (UK, Europe and Australia).
- Default prices: US average residential price, EIA 2026 forecast (checked Oct 2026); Ofgem price cap unit rate, 1 Oct – 31 Dec 2026, Direct Debit (VAT on electricity is 0% until 31 Mar 2027).
Estimates are for planning only and are not financial advice. Get written quotes before you buy.