Peak sun hours (PSH) is the single most important number in solar system sizing, and it's also the most commonly confused with something else: daylight hours. Understanding the difference — and knowing your own location's value — is what separates a system that meets your needs from one that quietly falls short every winter. This guide defines peak sun hours precisely, gives typical values for cities around the world, and explains how season, tilt, and orientation change the number for your specific roof.
What exactly is a peak sun hour?
One peak sun hour equals one hour of sunlight at an intensity of 1,000 watts per square meter — the standard reference intensity used to rate solar panels. In practical terms, 1 PSH is equivalent to 1 kWh of solar energy hitting each square meter of a surface.
Real sunlight intensity varies constantly through the day — weak at sunrise, strongest around midday, weak again near sunset, and reduced further by cloud, haze, and atmosphere. Peak sun hours compress all of that variable irradiance into an equivalent number of hours at full, standard intensity. A location with 5 peak sun hours receives the same total solar energy as 5 hours of constant, full-intensity sun, even though the actual daylight period is much longer and the intensity curve looks nothing like a flat line.
Peak sun hours vs. daylight hours
Daylight hours simply measure the time between sunrise and sunset, regardless of cloud cover or the sun's angle in the sky. A location can have 12 hours of daylight but only 5 peak sun hours, because much of that daylight period involves low sun angles or weak intensity. This is why "it's light outside for 14 hours" is not useful for solar sizing — what matters is the equivalent full-intensity hours, which is always lower, often by half or more.
Typical peak sun hours by city
| City | Approx. peak sun hours (daily average) |
|---|---|
| Phoenix, USA | 6.5 |
| Dubai, UAE | 6.0 |
| Los Angeles, USA | 5.6 |
| Karachi, Pakistan | 5.6 |
| Nairobi, Kenya | 5.4 |
| Delhi, India | 5.3 |
| Madrid, Spain | 5.0 |
| Sydney, Australia | 4.7 |
| Lagos, Nigeria | 4.3 |
| Seattle, USA | 3.7 |
| Berlin, Germany | 3.0 |
| London, UK | 2.9 |
These are annual daily averages — actual values swing above and below this through the year, and local cloud patterns, altitude, and air quality can shift a specific site from the regional average.
Seasonal variation
Peak sun hours change through the year almost everywhere, driven by day length and the sun's angle. A location with an annual average of 4 PSH might see 5.5–6 in summer and 2–2.5 in winter — a swing that matters most for off-grid systems, where winter is usually the limiting design case rather than the annual average. Grid-tied systems can rely on the annual average because summer surplus and winter shortfall roughly balance out over a year, but off-grid systems need to be sized around the worst month, not the average one, as covered in our off-grid sizing guide.
The effect of tilt and orientation
A panel's actual output depends on how directly sunlight hits its surface, which is why tilt and orientation change effective peak sun hours for a given roof:
- Optimal tilt is generally close to the site's latitude, though a flatter tilt trades some winter output for more summer output, and a steeper tilt does the reverse.
- Optimal orientation points panels toward the equator — south-facing in the northern hemisphere, north-facing in the southern hemisphere.
- East- or west-facing roofs typically lose roughly 15–20% of the output of an equator-facing roof at the same tilt.
- Flat roofs with no tilt lose some output versus an optimally tilted array, though less than you might expect at low latitudes.
Our solar panel angle calculator estimates how your specific roof pitch and orientation affect output compared to the optimal setup for your latitude.
How to find your peak sun hour value
A few reliable sources, roughly in order of convenience:
- This site's built-in dataset — our solar panel calculator uses location-based peak sun hour data automatically, so you don't need to look it up separately.
- PVGIS — a free tool covering Europe, Africa, and much of Asia, widely used by installers for irradiance data.
- NREL's PVWatts — the standard free tool for US locations, also usable for many international sites.
- National solar atlases — many countries publish their own solar resource maps, often through their energy or meteorological agencies.
If you're comparing sources, check whether the figure quoted is an annual average or a specific month — the two can differ by a factor of two or more at higher latitudes, and using the wrong one will throw off any sizing calculation downstream.
Why peak sun hours matter for every sizing calculation
Every core solar formula on this site depends on peak sun hours as an input. System size, panel count, and array size for off-grid systems all divide daily energy need by peak sun hours, adjusted for system losses — see the formula and worked examples in how many solar panels a 3-bedroom house needs. Getting this one number right, for your specific location and roof, is the foundation everything else is built on.