Solar panel angle in winter and summer
The best fixed angle, the best summer and winter angles, and what adjusting gains at seven latitudes.
Definitions
Irradiance, irradiation and peak sun hours
Two words sound alike. Irradiance is the power of sunlight on a surface at a moment, in watts per square metre. Irradiation is the energy collected over a time, in kilowatt-hours per square metre.
Solar panels are rated at an irradiance of 1,000 W/m², about the strongest midday sun on a clear day. A day’s irradiation divided by those 1,000 W/m² gives the peak sun hours: the number of hours of full sun that would deliver the same energy. Five kilowatt-hours per square metre in a day are five peak sun hours, however the light was spread out.
An example: in London on 21 June the sun is up for 16.6 hours, and a clear sky delivers 7.7 kWh to each square metre of flat ground, 7.7 peak sun hours. On 21 December the sun is up for 7.8 hours and a clear sky delivers just 0.7 kWh to the flat ground, but 2.5 kWh to a panel tilted 44° towards the south.
The numbers
Clear-sky peak sun hours by latitude and month
| Latitude and surface | January | March | June | September | December | Year |
|---|---|---|---|---|---|---|
| 30° N, flat | 3.6 | 5.9 | 7.9 | 6.3 | 3.3 | 2,137 kWh/m² |
| 30° N, tilted 28° | 5.4 | 6.9 | 7.0 | 7.0 | 5.1 | 2,373 kWh/m² |
| 40° N, flat | 2.3 | 4.9 | 7.9 | 5.5 | 2.0 | 1,860 kWh/m² |
| 40° N, tilted 37° | 4.6 | 6.5 | 7.0 | 6.7 | 4.2 | 2,237 kWh/m² |
| 50° N, flat | 1.2 | 3.8 | 7.7 | 4.5 | 0.9 | 1,541 kWh/m² |
| 50° N, tilted 44° | 3.3 | 6.0 | 7.2 | 6.3 | 2.8 | 2,035 kWh/m² |
| 60° N, flat | 0.3 | 2.6 | 7.3 | 3.3 | 0.1 | 1,216 kWh/m² |
| 60° N, tilted 50° | 1.6 | 5.1 | 7.2 | 5.7 | 1.1 | 1,741 kWh/m² |
In June a flat surface catches as much as a tilted panel, or more, because the sun is high. In winter the tilt multiplies the energy: at 50° north a panel at the best year-round angle collects three times as much in December as the flat roof under it. Over a year the tilt is worth 11% at 30° and 43% at 60°. The tilts are the clear-sky optimum for a panel facing due south; see how the best angle changes with the season.
From sunlight to electricity
Turning peak sun hours into kilowatt-hours
A panel’s rating in kilowatts-peak (kWp) is its output at 1,000 W/m². Its energy in a day is therefore roughly the peak sun hours times the rating, times a performance ratio for the losses in the inverter, the wiring, the heat and the dirt; 0.8 is a common figure for a good roof system, and the one this site uses.
A 4 kWp array at 50° north, tilted at 44°, on a clear June day: 7.2 × 4 × 0.8, about 23 kWh. On a clear December day: 2.8 × 4 × 0.8, about 9 kWh. Those are clear-sky ceilings; the real monthly averages are lower, as the next section explains.
The weather
Clouds bring the numbers down
Every figure above is for a cloudless sky. Clouds cut the direct beam and add some diffuse light from the rest of the sky, and over a month they bring the energy well below the clear-sky value, more in cloudy climates and in winter. On a day with half the light a clear sky would bring, a clearness of 0.5, London’s 7.7 kWh/m² on 21 June becomes about 5.8.
The solar panel angle calculator applies a typical clearness for each climate, from about 0.42 for cloudy maritime climates to 0.68 for deserts, as the method page describes. For a quote or a contract, use measured climate data such as the European Commission’s PVGIS.
A common confusion
Sunshine hours are something else
Weather services publish sunshine hours: the time during which the direct sun is strong enough to cast a clear shadow, defined by the World Meteorological Organization as a direct beam of more than 120 W/m². A summer day can have twelve sunshine hours and seven peak sun hours. The two are related, but you cannot turn one into the other without knowing how high the sun was during those hours.
Keep reading
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