What a peak sun hour measures
Solar panels are rated at a standard test intensity of 1,000 W/m². One peak sun hour is one hour of that full intensity — 1 kWh/m² of energy. Real sunlight ramps up in the morning, peaks around midday and fades in the evening; the peak sun hour figure compresses that whole curve into an equivalent number of full-intensity hours. A day with 4.5 peak sun hours delivered as much energy as 4.5 hours of perfect midday sun.
Why it is not daylight hours
An Estonian June day has about 18 hours of daylight but only around 5 peak sun hours — most daylight is far below test intensity. This is the single most common solar planning error: multiplying panel watts by daylight hours overestimates harvest three- to four-fold. Always multiply by peak sun hours instead, and then subtract system losses.
How to find your number
Use a real irradiance database rather than a rule of thumb: for Europe the European Commission's free PVGIS tool gives monthly figures for any point on the map, and the Global Solar Atlas covers the world. Look at the worst month you need the system to work in, not the annual average.
| Location and season | Peak sun hours / day |
|---|---|
| Northern Europe, summer | 4–5.5 |
| Northern Europe, winter | 0.5–1.5 |
| Central Europe, summer | 5–6 |
| Southern Europe, summer | 6–7.5 |
| Desert climates | 7–8.5 |
Using the number in calculations
Daily harvest = panel watts × peak sun hours × system efficiency. A 200 W panel at 4.5 peak sun hours and 75% system efficiency yields about 675 Wh per day. Refilling a half-empty 100 Ah / 12 V battery (600 Wh needed) then takes roughly 4 hours of peak sun — about 0.9 charging days. The Solar Charge Time calculator runs this live and lets you compare panel sizes; to check the consumption side of the balance, see the battery bank sizing guide.
Common mistakes
Using daylight hours instead of peak sun hours; planning the year on summer figures when the system must survive winter; ignoring shading, which removes far more than its shadow's share from a series string; forgetting that flat-mounted panels see fewer peak sun hours than tilted ones; and skipping system losses — controller conversion, wiring, temperature and battery charge efficiency together typically cost 20–30%.
Frequently asked questions
Do panels produce nothing outside peak sun hours?
They produce all day — peak sun hours are an accounting unit. A day of varying light is expressed as the equivalent number of hours at full rated intensity, so morning and evening output is already counted.
Why is winter so much worse than summer in the north?
Low sun angle, short days and cloud cover stack: Northern Europe drops from 4–5.5 peak sun hours in summer to 0.5–1.5 in winter — often a five-to-tenfold difference in daily harvest.
Does panel tilt and orientation change my peak sun hours?
Effectively yes. Databases like PVGIS report irradiation on the plane of your array, so a well-tilted south-facing panel sees more peak sun hours than a flat one at the same location.