Peak sun hours explained
One peak sun hour is one hour of sunlight at an intensity of 1,000 watts per square meter, the "full sun" panels are rated under. A location with 4 peak sun hours receives the same total solar energy in a day as four hours of full sun, even if daylight lasts twelve.
If you're sizing solar panels, this is the number that turns a panel's rated wattage into a realistic daily energy estimate. It's also widely misunderstood.
Peak sun hours are not daylight hours
At sunrise and sunset, sunlight hits panels at a low angle through more atmosphere, so it's weak. Around midday, it's strongest. Peak sun hours add up all of that varying sunlight and express it as the equivalent number of hours at full intensity.
A summer day might have 14 hours of daylight but only 6 peak sun hours. A winter day in a northern city might have 9 hours of daylight and fewer than 2 peak sun hours. This is why the same panels can produce three times as much in July as in December.
How peak sun hours are used
A 200 W panel at a site with 5 peak sun hours, with a 0.75 factor for heat, dust, wiring and controller losses: 200 × 5 × 0.75 = 750 Wh per day. Flip the formula around and you get the panel size needed for a given load. That's exactly what the solar panel size calculator does.
Typical ranges
These are broad planning ranges for panels tilted reasonably toward the sun. Local weather and terrain matter a lot, so look up your actual location for anything important.
| Conditions | Approximate peak sun hours/day |
|---|---|
| Cloudy climates or high latitudes in winter | 1–2.5 |
| Much of the continental US, annual average | 3.5–5.5 |
| Desert Southwest US, summer | 6–7+ |
How to find yours (free tools)
- NREL PVWatts (pvwatts.nrel.gov), from the US National Renewable Energy Laboratory. Enter an address and it estimates monthly solar output, including the solar radiation (kWh/m²/day) figures, which are numerically the same as peak sun hours.
- Global Solar Atlas (globalsolaratlas.info), backed by the World Bank Group, gives solar resource data for locations worldwide.
Use the monthly values, not just the annual average. If you depend on solar in winter, plan with your lowest month.
What reduces real output below the estimate
- Heat. Panels lose efficiency as they get hotter, so a scorching summer day doesn't produce as much as the sunshine suggests.
- Angle and orientation. Portable panels lying flat on the ground, or on a flat RV roof, catch less than panels tilted toward the sun, especially in winter when the sun is low.
- Shade. Even partial shade on part of a panel can cut its output sharply.
- Dust, snow and dirt. Wipe panels occasionally, especially in dusty areas.
- Charge limits. If your power station's solar input maxes out at 200 W, a 400 W array won't charge it any faster.
A quick example
You use about 1,000 Wh per day in a van. In summer you get around 6 peak sun hours, and in October around 3.5. With flat-mounted panels (0.65 factor):
- Summer: 1,000 ÷ (6 × 0.65) ≈ 256 W of panels.
- October: 1,000 ÷ (3.5 × 0.65) ≈ 440 W of panels.
Same van, same use, but nearly twice the panel wattage needed in autumn. That gap is why understanding peak sun hours matters before you buy.