Solar Panel Calculator
Sizes a solar array from your actual consumption. Enter your daily kWh and the peak sun hours for your location, and it works out the array size, the panel count and the roof area needed.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
array kW = daily kWh ÷ (peak sun hours × efficiency) panels = array kW × 1000 ÷ panel watts
Peak sun hours already fold the whole day into an equivalent number of hours at full irradiance, which is why the formula looks too simple. The efficiency term covers everything between the panel rating and the meter: inverter, wiring, heat, dirt and mismatch.
What to watch out for
Four things decide the answer:
- Peak sun hours, not daylight hours. A location with 4 peak sun hours might have 14 hours of daylight — the figure is the equivalent hours at full 1,000 W/m². Roughly 3 in northern Europe, 4–5 in the southern UK or northern US, 5–6 in the US southwest or most of India.
- Losses are larger than people expect. Panels are rated in laboratory conditions at 25 °C. Real arrays run hot, get dirty, and lose more in the inverter and cabling. 20% is a realistic long-run figure.
- Annual average hides the seasons. A system sized to cover your yearly use will overproduce heavily in summer and fall well short in winter. Grid-tied that is fine; off-grid it is not, which is why off-grid systems are sized on the worst month.
- Orientation and shade are not in this figure. A north-facing roof in the northern hemisphere, or an array shaded for part of the day, can lose 20–40% beyond the loss factor here.
Frequently Asked Questions
How many solar panels do I need?
For 10 kWh a day at 4 peak sun hours with 20% losses, about 3.1 kW of array — roughly 8 panels at 400 W. Your own consumption and sun hours change it substantially.
What are peak sun hours?
The equivalent number of hours per day at full 1,000 W/m² irradiance. It bundles the whole day's energy into one number, so a site with 4 peak sun hours receives 4 kWh/m² per day.
How much roof space does solar need?
About 1.9 m² per 400 W panel, so roughly 5 m² per kW. A 4 kW system needs around 20 m² of usable, unshaded roof.
Should I size for 100% of my usage?
Grid-tied, sizing to 100% of annual use is common — summer surplus offsets winter shortfall. Size above 100% if an EV or heat pump is coming, and below it if export rates are poor.