One peak sun hour is one hour of solar irradiance at 1,000 W/m² — the standard test condition panels are rated at.
So a location with 5 peak sun hours receives 5 kWh/m² of solar energy in a day. It might have 14 hours of daylight; the 5 is the energy total expressed as an equivalent number of full-strength hours.
That is why panel arithmetic is so simple once you have the figure: a 400 W panel in a 5 PSH location produces about 2 kWh a day before derates.
Typical annual averages
| Region | Peak sun hours/day |
|---|---|
| Pacific Northwest | 3.0–3.7 |
| Northeast, Great Lakes | 3.5–4.2 |
| Midwest | 4.0–4.7 |
| Mid-Atlantic, Southeast | 4.3–5.0 |
| Texas, Southern Plains | 4.8–5.5 |
| Mountain West | 5.0–6.0 |
| Desert Southwest | 5.8–6.5 |
Nearly a factor of two from top to bottom. The same array in Arizona and in Seattle are different investments.
The annual figure hides the swing
December production across most of the US runs 40 to 60% of the annual average, and the further north the sharper it gets.
| Seattle | Phoenix | |
|---|---|---|
| Annual average | 3.6 | 6.3 |
| June | 6.0 | 7.6 |
| December | 1.2 | 4.4 |
| Winter as % of annual | 33% | 70% |
Grid-tied, that swing does not matter much — the grid covers December. Off-grid it is the number that sizes the whole system, and using the annual average instead is the single most common off-grid design error.
Where to get a real figure
NREL's PVWatts calculator uses measured typical-meteorological-year data for your actual coordinates and lets you enter tilt and azimuth. It is free, it is the source most of the industry uses, and it gives monthly output rather than an annual average.
A table by state is a starting point. A figure for your latitude, longitude, tilt and orientation is an input.
What it does not include
Peak sun hours describe the resource arriving at a surface. Everything that happens after that — heat, soiling, wiring, inverter efficiency — is the derate factor, and 0.77 is the standard default.
Multiplying peak sun hours by panel wattage without the derate overstates production by about a quarter. That error is why so many systems that "should" cover the bill do not.
Run your own numbers on the system sizing calculator, using the December figure rather than the annual one if the system has to stand alone.