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One peak sun hour is one hour of sunlight at an intensity of 1,000 watts per square metre. If your location receives 4.5 kWh of solar energy per square metre on an average day, it gets 4.5 peak sun hours, even though the sun may be up for 12 or 14 hours.
Peak sun hours are the single most important location number in solar sizing. Get this wrong and every other calculation is wrong.
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Why we use peak sun hours
Sunlight intensity changes all day. At dawn it might be 100 W/m², at noon 900 W/m², then back down at dusk. Adding up all of that energy across the day and expressing it as “hours at full strength” makes the math simple, because solar panels are rated at exactly 1,000 W/m².
So a 400 W panel under 4.5 peak sun hours receives the same energy as if it sat in perfect 1,000 W/m² sunshine for 4.5 hours, producing a theoretical 1,800 Wh before losses.
Daylight hours vs peak sun hours
| Situation | Daylight hours | Peak sun hours |
|---|---|---|
| Arizona, June | 14 | about 7 |
| Texas, annual average | 12 | about 5 |
| New York, annual average | 12 | about 4 |
| London, annual average | 12 | about 2.7 |
| London, December | 8 | under 1 |
Daylight hours overstate your solar resource by a factor of two to four.
Typical annual averages by region
These are rough yearly averages on a south-facing panel tilted near latitude. Always check your exact location.
| Region | Typical peak sun hours |
|---|---|
| US Southwest (Arizona, Nevada, New Mexico) | 6.0 – 7.0 |
| California, Texas, Florida | 5.0 – 6.0 |
| US Midwest and Northeast | 3.8 – 4.8 |
| Pacific Northwest | 3.0 – 4.0 |
| United Kingdom | 2.5 – 3.0 |
| Germany | 2.8 – 3.2 |
| Spain, southern Italy | 4.5 – 5.5 |
| Australia (most cities) | 4.5 – 6.0 |
| Canada (Toronto, Calgary, Vancouver) | 3.5 – 4.5 |
How to find your exact peak sun hours
The most reliable free sources are:
- NREL PVWatts (pvwatts.nrel.gov) for the US and many other countries. It reports monthly solar radiation in kWh/m²/day, which equals peak sun hours.
- The EU’s PVGIS tool (re.jrc.ec.europa.eu/pvg_tools) covers the UK, Germany and the rest of Europe, plus most other regions, including Australia and the Americas.
- NASA POWER data for anywhere in the world.
Look at the monthly values, not only the annual average. For off-grid systems you should design around your lowest month.
Which number to use in your calculation
- Grid-tied home: use the annual average, because the grid covers seasonal shortfalls.
- Off-grid, year-round: use the worst month (often December or January in the northern hemisphere).
- Summer-only cabin or RV: use the average of the months you are actually there.
Peak sun hours in the formula
Array watts = Daily watt-hours ÷ (Peak sun hours × 0.80)
Example: a cabin using 5 kWh per day in a location with 3.5 winter sun hours needs 5,000 ÷ (3.5 × 0.80) = 1,786 W of panels. With 5.5 summer sun hours the same cabin would only need 1,136 W.
Try different values in our solar and battery calculator to see how location changes your system.
Worked example: same cabin, four locations
A cabin uses 4 kWh per day. Here is the array it needs in each place, sized for the annual average and for the worst month:
| Location | Annual peak sun hours | Array (annual) | December peak sun hours | Array (December) |
|---|---|---|---|---|
| Phoenix, Arizona | 6.5 | 769 W | 4.5 | 1,111 W |
| Dallas, Texas | 5.3 | 943 W | 3.6 | 1,389 W |
| Seattle, Washington | 3.5 | 1,429 W | 1.2 | 4,167 W |
| London, UK | 2.9 | 1,724 W | 0.7 | 7,143 W |
Array = 4,000 ÷ (sun hours × 0.80). In Seattle, designing for December almost triples the array. In London it more than quadruples it. That is why off-grid systems in cloudy climates rely on generators for the darkest weeks.
How to read PVWatts or PVGIS output
- Enter your address or click the map.
- Set tilt and azimuth (direction) to match your planned installation.
- Look for “Solar radiation (kWh/m²/day)” or monthly irradiation figures. For PVGIS monthly data, divide monthly kWh/m² by the number of days in the month.
- That number is your peak sun hours for each month.
- Use the lowest month for off-grid, the annual average for grid-tied.
Peak sun hours change with tilt
The same location gives different peak sun hours depending on how panels are mounted:
| Mounting (mid-latitude example) | Relative winter sun hours | Relative annual sun hours |
|---|---|---|
| Flat (0°) | about 60% | about 88% |
| Tilt = latitude | 100% (reference) | 100% |
| Tilt = latitude + 15° | about 110% | about 97% |
Approximate; the exact numbers depend on latitude and climate.
Common mistakes
- Using hours of daylight or “hours of sunshine” from weather sites.
- Using a city-wide average when your site has hills, trees or buildings that shade early and late.
- Forgetting that RV panels are usually flat and lose winter sun hours.
Quick reference: peak sun hours for selected cities (annual average, tilted panels)
| City | Approx. peak sun hours |
|---|---|
| Phoenix | 6.6 |
| Perth | 5.8 |
| Los Angeles | 5.8 |
| Dallas | 5.3 |
| Miami | 5.3 |
| Brisbane | 5.3 |
| Sydney | 4.9 |
| Chicago | 4.5 |
| Calgary | 4.5 |
| Boston | 4.1 |
| Toronto | 3.9 |
| Munich | 3.4 |
| London | 2.9 |
| Manchester (UK) | 2.6 |
Approximate values for planning; check PVGIS or PVWatts for your exact site, tilt and direction.
FAQ
Is a peak sun hour the same as a sunny hour?
No. A bright winter hour with the sun low in the sky may only deliver 400 W/m², which counts as 0.4 peak sun hours.
Does panel tilt change peak sun hours?
Yes. Published figures usually assume an optimal tilt. Flat-mounted panels, like many RV setups, receive fewer effective sun hours, especially in winter. See solar panel tilt angle.
What about cloudy climates?
Clouds are already included in the averages. That is why the UK averages under 3 peak sun hours despite long summer days. Read do solar panels work on cloudy days.
What are peak sun hours in Canada and Australia?
Canadian cities average roughly 3.5 to 4.5 peak sun hours: Vancouver about 3.5, Toronto 3.9 and Calgary 4.5. Australian capitals sit higher, from Melbourne at about 4.3 to Sydney at 4.9 and Perth at 5.8. Check PVGIS or PVWatts for your town.
Is 4 peak sun hours good for solar?
Yes. Most of Germany and the UK get under 3, and solar still makes sense there. Four or more is a good resource.
Related guides
- What Can a 1kW Solar System Run? kWh per Day for Cabins, RVs, Sheds and Backup
- Solar Panel Efficiency Explained (and When It Matters)
- What Can a 1kW to 20kW Solar System Run? kWh per Day and Appliance List
- What Can a 3kW Solar System Run? kWh per Day, Appliances and Battery Backup
Sources and further reading
- NREL PVWatts Calculator: solar output estimate for any address.
- European Commission JRC: PVGIS: free solar output and optimum-tilt tool for Europe and most of the world.
- Global Solar Atlas (World Bank): sun data and solar potential for any location.
- Natural Resources Canada: Photovoltaic Potential Maps: solar potential for Canadian municipalities.