Vicaya GlobalCalculator · Solar · reference
Peak sun hours, and what they actually mean
Peak sun hours are not hours of sunshine — they are a day’s energy dressed up as a duration. Here is the figure for your location and panel angle, and the arithmetic it unlocks.
12 × 450 W at 21% is 2.14 m² per panel — about 26 m² of roof.
A peak sun hour is not an hour of sunshine. It is the day's energy divided by 1,000 W/m² — how long full noon-strength sun would have taken to deliver the same amount.
In London, United Kingdom, a panel on this plane collects 3.22 peak sun hours on an average day across the year. December gives 0.81 and July 5.31 — a ratio of 6.5 to one, even though the days themselves are nothing like 6.5 times longer.
Peak sun hours by month
| Month | h |
|---|---|
| Jan | 0.98 |
| Feb | 1.63 |
| Mar | 2.97 |
| Apr | 4.59 |
| May | 4.86 |
| Jun | 5.24 |
| Jul | 5.31 |
| Aug | 4.60 |
| Sep | 3.90 |
| Oct | 2.33 |
| Nov | 1.40 |
| Dec | 0.81 |
| Average | 3.22 |
The conversions everything else is built on
| Array size | 5.40 kWp | 12 panels × 450 W ÷ 1,000 |
|---|---|---|
| Output on an average day | 17.4 kWh | kWp × peak sun hours. This is the identity the unit exists for |
| Output in Dec | 4.4 kWh | the figure that sizes an off-grid system |
| Battery energy | 1,200 Wh | 100 Ah × 12 V. Amp-hours alone say nothing |
| Time to fill it | 0.26 h | of full-strength sun, at 85% charging efficiency |
| Days to fill it in Dec | 0.32 | the same energy, spread over the days that month actually provides |
Why the unit is useful, and where it misleads
Panels are rated at 1,000 W/m². That is the whole trick: because a peak sun hour is one hour at exactly that intensity, a panel's watt rating multiplied by the day's peak sun hours gives watt-hours directly, with no other arithmetic. Nearly every off-grid rule of thumb in circulation rests on that identity, usually without mentioning it.
Where it misleads is in sounding like a duration. Four peak sun hours in June is roughly five hours of strong sun; four peak sun hours in December, if you could get them, would be most of a short day of thin light. The unit collapses intensity and duration into one figure, which is convenient for arithmetic and quietly wrong for intuition — and it is why a system sized on an annual average peak sun hour figure fails in winter.
What these figures are and are not
- Plane-of-array, not horizontal. Change the tilt or bearing above and every figure moves. A table that does not tell you which plane it describes cannot be used safely.
- Long-run averages, not this year. The climate behind them is PVGIS multi-year data. Any individual month can be far better or worse.
- Irradiance on the glass, before the panel touches it. Peak sun hours say nothing about heat, dirt, shade or conversion — a panel in 40°C heat delivers noticeably less per peak sun hour than the same panel in the cold.
- No shading. The plane is treated as seeing an unobstructed sky.
Frequently asked questions
What are peak sun hours?
Peak sun hours are the number of hours a day that sunlight averages 1,000 watts per square metre — the intensity a panel is rated at. A twelve-hour day delivers only a few peak sun hours, because the sun is weaker at the edges of the day. It is the figure every solar generation estimate is built on.
Pick a location and the page shows its peak sun hours, so you can calculate peak sun hours where you live rather than use a national average.
What is the difference between peak sun hours and daylight hours?
Daylight hours are how long the sun is above the horizon; peak sun hours are how much full-strength energy actually arrives. This calculator gives the peak sun hours for your own plane — your tilt and bearing — not a flat horizontal table.
How many peak sun hours do I get at my zip code or postcode?
It depends on your latitude, your climate, and how the panels are angled. The tool computes the figure for your location and array, along with the watt-hour and amp-hour conversions built on it.