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.

Location

Where are you?

Searches use OpenStreetMap. Nothing is stored.

Or pick an example location

Or enter coordinates directly

Latitude

Longitude

Searching or using your location fills these in, so you can always see exactly which point the figures are for.

Climate comes from PVGIS, the European Commission's reference dataset — the same source this model is validated against, for examples and searches alike.

The plane

Peak sun hours depend on which way the panel points. A horizontal table is a different number.

Which way does the roof face?

The direction you would face standing on the roof looking down the slope.

How steep is the roof?

Measured up from flat. Most pitched roofs are between 30 and 45 degrees.

Your array

How many panels

Type any number — there is no upper limit.

Watts per panel

The number on the label. Watts times peak sun hours is watt-hours a day.

A battery to charge

Battery capacity

Amp-hours, the number batteries are sold by.

Battery voltage

Volts. Amp-hours mean nothing without it — 100 Ah at 12 V is a quarter of 100 Ah at 48 V.

Charging efficiency %

Controller, wiring and what the battery refuses to accept.

3.22peak sun hours, yearly average

5.31in Jul, the best month

0.81in Dec, the worst

17.4 kWha day on average, from 5.40 kWp

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

On the plane described, not on the horizontal. Most published peak sun hour tables are horizontal figures, which understate a well-tilted panel — sometimes by a great deal in winter, when tilt matters most.

Peak sun hours, by month
Month	h
Jan	1
Feb	2
Mar	3
Apr	5
May	5
Jun	5
Jul	5
Aug	5
Sep	4
Oct	2
Nov	1
Dec	1
Total	39

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.