Peak sun hours by city (and what they mean)

What a peak sun hour is

Solar panels are rated at 1,000 watts of sunlight per square metre — full, clear midday sun. A peak sun hour is one hour of that. Real days start dim, peak around noon and fade, so the day’s total sunlight is expressed as the number of full-sun hours that would deliver the same energy. A day with 5 peak sun hours puts as much energy on the panel as five hours at 1,000 W/m², even if the sun is up for fourteen.

Numerically, peak sun hours equal the day’s solar energy in kilowatt-hours per square metre (kWh/m²/day). That is why solar maps and PVWatts report sunlight in kWh/m²/day — it is the same number.

panel Wh per day ≈ panel watts × peak sun hours × (1 − system losses)

Peak sun hours for example cities

Two things stand out. First, the spread between places: Phoenix gets about 70% more yearly sun on a tilted panel than Seattle. Second — and more important for off-grid systems — the spread between months: Seattle’s best month has nearly four times its worst. A system sized on Seattle’s yearly average runs short every winter.

Which number to use

• Grid-tied home solar: the yearly figure (or better, PVWatts’ yearly kWh) — the grid evens out the seasons.
• Off-grid, year-round: the lowest month you need power in. That is why off-grid arrays look oversized in summer.
• Summer-only cabin, RV or van trip: the months you are actually there.
• A flat panel on a van roof: less than these tilted figures, especially in winter, when the low sun hits a flat panel at a glancing angle.

To see what the numbers mean in practice: how much power a 100W solar panel produces per day shows one panel at 4.5 sun hours, in Phoenix and in a Seattle December; how many solar panels to charge a 100Ah battery shows how the panel count swings with the same numbers; and how many solar panels for 1000 kWh per month uses the kWh-per-kW column for a home system.

How to look up your own peak sun hours

1. Open NREL’s PVWatts calculator (now hosted by the National Laboratory of the Rockies at pvwatts.nlr.gov) and enter your address.
2. Keep the default system: 1 kW, standard module, fixed (open rack). Set the tilt to your panel’s angle — your latitude if you are not sure — and the azimuth to 180° for due south.
3. Run it and open the monthly results. The Solar Radiation (kWh/m²/day) column is your peak sun hours for each month.
4. Use the annual figure for grid-tied sizing, or your worst month for off-grid. The AC Energy column, per kW of panels, is what the solar system size calculator calls kWh per kW per year.

Why these numbers aren’t guarantees

PVWatts uses “typical meteorological year” data — a statistically typical year built from many years of satellite and ground measurements. Any real year can be sunnier or cloudier, and any real week can be much worse than the monthly average. Local shade from trees and buildings isn’t in the data at all. Treat the table as a planning figure and keep a margin — or a generator — for the bad weeks.

Peak sun hours vs daylight hours

A June day in Seattle has around sixteen hours of daylight, but PVWatts gives it under six peak sun hours on a tilted panel. Early and late light is weak, clouds cut the rest. Never size a system on hours of daylight.