Peak sun hours are the hours per day of full-strength sunlight a solar panel receives, once all the weak morning and evening light is squeezed together. Depending on where you live, average peak sun hours run from about 2.5 to 6.5.
That one number decides how much solar power your roof can make, and it swings a lot from place to place. The same panel in Arizona catches more than twice as much sunlight as the same panel in Scotland. Same brand, same wires, same install. Only the sun is different.
What Are Peak Sun Hours?
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One peak sun hour is one hour of direct sunlight at full intensity, about 1000 watts on every square meter. It's a way to measure the total amount of solar energy a spot receives in a day, not how long the sun is up.
Solar people don't count regular daylight hours, because daylight is uneven. A weak winter morning might give you 200 watts per square meter. The sun is up, but that hour only counts as one-fifth of a peak hour.
Think of it like filling a bucket with a hose. A trickle in the morning, a strong stream at noon, a trickle again in the evening. Your sun hours tell you how long the hose would need at full blast to fill the same bucket.
So a day with 12 hours of daylight might only add up to 5 peak sun hours.
Curious what a panel does with all that light? Here's how sunlight becomes electricity.
Average Peak Sun Hours By Region And State
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These yearly averages come from PVGIS, the free solar map run by the European Commission. They're for flat ground. In Europe and the US, a solar panel tilted toward the sun catches about 10 to 20% more.
North Africa. About 6.5 hours per day in Aswan, Egypt, some of the sunniest land on Earth. Accra in Ghana gets close to 6.
Inland Australia. About 6 in Alice Springs.
US states. Arizona and southern California get 5.5 to 6, while the state of Texas gets a bit under 5. Peak sun hours by state are a handy first guide, but a sunny state still has cloudy corners.
Southern Spain. About 5 in Seville, one reason the region is a solar farm hotspot.
Netherlands, Germany, UK. 2.5 to 3. Berlin and the middle of the Netherlands get about 3, Manchester about 2.6.
Scandinavia. Stockholm gets about 2.8, but nearly all of it arrives in summer.
Compare southern Spain with Manchester and the Spanish roof receives about twice the sunlight. That's why 10 solar panels in Seville might cover everything a home uses, while the same 10 in Manchester cover about half.
How Sun Hours Change With The Seasons
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Yearly averages hide a big catch. Solar isn't close to balanced across the year.
In the Netherlands, a south-facing panel at the best angle gets about 5 sun hours per day in June and about 1 in December. That's a five-times swing. In Stockholm it's worse: about 6 in June and only half an hour in December, when short days and thick cloud team up.
Near the equator the seasons barely matter. In Accra, the darkest month still gets close to 5 and the brightest about 6.5.
This is important if you're going fully off-grid. In summer you'll generate more electricity than you can use, and in the dark half of the year you might run short. You need batteries, a backup, or both, and you plan for the worst month, not the average.
→ What to think about before going off-grid with solar
Peak Sun Hours Vs Effective Sun Hours
No solar system turns every bit of sunlight into power at the plug. Heat, dirt and cables each take a small bite, and so does the inverter, the box that changes solar power into the kind your house uses.
That's why solar tools often show a second, lower number: "effective sun hours", also called daily yield. It's what 1 kW of solar actually delivers per day once those losses are taken into account, so it's the better figure to understand your real output.
For a well-aimed Dutch roof, PVGIS gives about 3.5 peak sun hours a day but about 2.8 effective ones. Roughly 20% of the potential disappears on the way.
Our yearly kWh calculator uses the effective kind, so its estimate of your solar output is closer to what your meter will show.
How To Find The Number For Your Location
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The averages above are a rough guide, and your specific location can be quite different. Three free tools give you a more accurate number.
PVGIS. Type in your address and it shows the average peak sun hours your roof receives in every month, based on years of satellite weather data. It covers Europe, Africa and many other parts of the world.
PVWatts. The US National Renewable Energy Laboratory's calculator does the same for any American location and turns it into an energy estimate for your system.
SunCalc. It doesn't count sun hours, but it shows the path the sun takes over your house on any day of the year. Handy for spotting which tree or chimney will throw shade, and when.
What Affects Your Sun Hours
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A few factors can push the sunlight on your roof below the average for your location. Most of them you can spot before you design anything.
Shade. A tree, a chimney or a tall neighbour can block the sun for hours a day, and even shade on one corner of a panel can drag down a whole array. Here's what shade does to solar panels.
Tilt. A flat panel catches less than a tilted one. In the Netherlands, PVGIS shows a flat solar array making about 16% less than one tilted at 35 degrees and facing south. Our roof angle guide shows how to find the best tilt for your place.
Direction. In the northern hemisphere, south-facing roofs catch the most. East or west still work but give about 20% less, and north is mostly a no.
Local weather. A coastal town with lots of fog or low cloud gets less than the map suggests, and a high town with clear skies can get more.
Why Sun Hours Are Important For Sizing
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Once you know your peak sun hours, a rough size for your solar system is one line of math.
Daily energy use (kWh) ÷ peak sun hours = array size (kW)
For example, use 10 kWh a day where you get 4 hours, and you need about 2.5 kW of solar. The same 10 kWh with 2 hours needs 5 kW. Twice as many panels, for the same fridge and lights.
Not sure what you use per day? Start with working out your solar energy needs in kWh. Then how many panels you need to go off-grid walks through the rest, or the solar panel calculator does the math for your location.
Panel Efficiency, The Other Half Of The Sum
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Sun hours tell you the amount of sunlight that arrives. Efficiency tells you how much of that amount a solar panel turns into electricity.
Monocrystalline turns about 20% of the sunlight into power. Polycrystalline and most thin-film usually manage a bit less.
So the full sum is: peak sun hours × panel area × efficiency = energy per day.
A small, efficient solar array in a sunny place can beat a much bigger one under cloudy skies. The sun and the panel both count, and the best solar system gets both right.
Sources
- European Commission JRC, PVGIS photovoltaic geographical information system
- PVEducation, Average solar radiation
- Fraunhofer ISE, Photovoltaics Report
- SunCalc, sun position and sunlight phases
Hard Words
| Term | What it means |
|---|---|
| Peak sun hour | One hour with sunlight at full strength, about 1000 watts per square meter. Different from one hour of daylight, which can be much weaker. |
| Effective sun hours | Peak sun hours minus the normal losses from heat, dirt, cables and the inverter. Also called daily yield. |
| Intensity | How strong the sunlight is at one moment. Low intensity in the morning, high intensity at noon. |
| PVGIS | A free solar map from the European Commission. Shows monthly sun hours for any address it covers. |
| PVWatts | A free US solar calculator from the National Renewable Energy Laboratory. |
| Seasonal swing | The change in sun hours between summer and winter. In the Netherlands, about five times. |
| Array size | The total kilowatts of solar you need. Your daily kWh use divided by your peak sun hours. |
Common Questions
What are peak sun hours?
The hours per day of full-strength sunlight a spot receives once all the weaker light is added together. A day with 12 hours of daylight might only add up to 5.
How many peak sun hours does my area get?
On flat ground, about 6.5 in southern Egypt, 6 in inland Australia, 5.5 to 6 in Arizona and southern California, 5 in southern Spain and 2.5 to 3 in the Netherlands, Germany and the UK. PVGIS or PVWatts give the figure for your address.
Why do the dark months matter most?
A fully off-grid system has to get through the worst month, not the average one. In the Netherlands, December gets about a fifth of the sun June does.
How do I use peak sun hours to size a solar system?
Divide your daily kWh use by your peak sun hours. 10 kWh a day with 4 hours needs about 2.5 kW of solar.