Quick answer
A peak sun hour is one hour of sunlight at 1,000 W/m² — the industry-standard unit that converts panel watts into daily energy. Most US locations get 3.5–6.5 peak sun hours per day on an annual average: the Southwest sits at 6+, the Sun Belt around 5–6, the Midwest and Mid-Atlantic around 4.2–4.9, and the Northwest and Northeast around 3.5–4.5. The number that actually decides your panel count is your worst month (winter typically runs 50–65% of the annual average), not the brochure average. Use the regional table below, then your exact location’s number from NREL’s PVWatts tool.
How to read this page: regional bands below are drawn from the US Energy Information Administration’s solar-resource maps (eia.gov, based on NREL’s National Solar Radiation Database — retrieved 2026-09-05) and rounded conservatively into bands; for any specific address, NREL’s PVWatts calculator (pvwatts.nrel.gov) is the authoritative source and takes thirty seconds. This site tests nothing and sells nothing on this page — the number is the product.
Why peak sun hours (not daylight hours)
A panel’s rated watts are measured at 1,000 W/m² of irradiance. Morning light, dusk, clouds, and winter angles all deliver less. “Peak sun hours” collapses all of that into one number: how many hours of full-strength sun your location averages per day. Daily energy is then simple:
Panel watts × peak sun hours × ~0.75 system efficiency ≈ daily watt-hours
Worked: a 400W panel at 4.5 peak sun hours → 400 × 4.5 × 0.75 ≈ 1,350Wh/day (annual-average conditions). The full method and the efficiency factors live in our panel output guide.
Regional peak-sun-hour bands (annual average)
| Region (representative states) | Annual avg PSH/day | Winter (worst-month) band | Summer band |
|---|---|---|---|
| Desert Southwest (AZ, NV, NM, inland CA) | 6.0–7.0 | 4.5–5.5 | 7.0–8.0 |
| Sun Belt (TX, OK, southern CA coast, FL south, GA, LA) | 5.0–6.0 | 3.8–4.8 | 6.0–7.0 |
| Mid-Atlantic / Midwest (VA, NC north, OH, IN, MO, KS, CO front range) | 4.2–4.9 | 2.7–3.6 | 5.2–6.2 |
| Northeast (NY, PA north, New England, MI, WI) | 3.8–4.5 | 2.2–3.0 | 4.8–5.8 |
| Pacific Northwest / cloud-belt (WA west, OR west, AK) | 3.3–4.2 | 1.6–2.4 | 4.5–5.8 |
| Mountain-high (UT, WY, ID, MT high country) | 4.8–5.8 | 3.0–4.2 | 6.0–7.2 |
Reading notes: bands are annual averages for typical locations in each region — a valley fog pocket or a high south-facing slope moves you within (or past) the band, which is why PVWatts-by-address is the final word. Alaska spans the widest range in the country. Florida’s summer monsoon season trims its summer band despite the latitude.
Peak sun hours calculator (watts → daily Wh, seasonal)
The calculator applies the site-standard 0.75 system-efficiency factor (controller, wiring, heat) and shows the winter band at 60% of your entered sun hours — plan array size on the winter row if you need year-round power. The full output calculator adds panel count and climate presets.
How the number changes your panel count
Two worked examples, same 2,000Wh/day cabin load (0.75 efficiency):
- Phoenix-class (5.5 annual, 4.8 winter): winter needs 2,000 ÷ (4.8 × 0.75) ≈ 556W of panel.
- Seattle-class (3.8 annual, 2.0 winter): winter needs 2,000 ÷ (2.0 × 0.75) ≈ 1,333W — more than double the array for the same winter power.
That is the entire argument for knowing your worst-month number before buying anything: it can double your panel budget. The complete load-planning method is our system sizing guide, and the seasonal-tilt side of the same problem is in the panel angle calculator.
Frequently Asked Questions
What is a peak sun hour exactly?
Why does my state's number come as a range?
Should I size my system on annual average or winter sun hours?
Do solar panels still produce on cloudy days?
Next logical reads
Solar panel output calculator How to size a solar system Panel angle calculator How to calculate your solar load