What Are Peak Sun Hours? The Number That Actually Sizes a Solar System

Ask how much sun a city gets and you’ll usually hear an answer in hours of daylight. Ask a solar installer the same question and they’ll answer with a different unit entirely: peak sun hours. The two numbers can disagree wildly — Seattle gets nearly 16 hours of daylight at the June solstice but averages fewer than 4 peak sun hours per day across the year — and the difference between them is the single most common source of confusion when people first size a solar system.

Peak sun hours are worth understanding even if you never put a panel on your roof, because they’re the honest measure of how much solar energy a place actually receives. Daylight duration tells you when the sun is up. Peak sun hours tell you what it delivers.

One Peak Sun Hour Is a Precise Amount of Energy

A peak sun hour is defined as one hour of sunlight at an intensity of 1,000 watts per square meter. That’s not an arbitrary number: 1,000 W/m² is roughly the intensity of direct sunlight falling on a surface facing the sun on a clear day, and it’s the reference irradiance used in Standard Test Conditions — the laboratory setup under which every solar panel’s nameplate wattage is measured (along with a 25°C cell temperature and a standardized atmospheric filter called AM1.5).

So a “400-watt” panel produces 400 watts when receiving one peak sun’s worth of light. One peak sun hour delivers one kilowatt-hour of solar energy to each square meter of surface. A location rated at 5 peak sun hours per day receives 5 kWh/m² of solar energy daily — whether that arrives as 5 actual hours at full blast or 10 hours at half intensity.

That last point is the key. The sun almost never delivers 1,000 W/m² for hours on end. Intensity ramps up from zero at sunrise, peaks around solar noon, and ramps back down to zero at sunset, tracing a curve across the day. Peak sun hours are the area under that curve, compressed into an equivalent number of full-intensity hours. It’s the same trick as measuring rainfall in inches rather than describing every shower.

Why 14 Hours of Daylight Might Equal 7 Peak Sun Hours

Two things separate daylight hours from peak sun hours: the sun’s angle and the atmosphere in its way.

When the sun sits low in the sky, its light strikes the ground obliquely, spreading the same energy over a larger area — the same geometry that makes winter sun feel weak and low-angle mornings gentle on the skin. A panel lying flat receives dramatically less energy from a sun at 15° altitude than from one at 60°.

Low sun also travels through far more atmosphere. At 30° altitude, sunlight crosses roughly twice the air mass it does from directly overhead; near the horizon, ten times or more. Every extra mile of air scatters and absorbs energy before it reaches the ground.

Put together, the first and last couple of hours of any day contribute almost nothing to the energy total. A 14-hour summer day in Denver might deliver 7 peak sun hours; a 9.5-hour December day in the same spot delivers barely 3. Day length and energy are related, but the exchange rate changes constantly with the sun’s height — which is why the noon sun altitude matters as much as the sunrise and sunset times.

How Many Peak Sun Hours Does the US Get?

Averaged over the year and accounting for real weather, the continental US spans roughly a factor of two:

Region	Typical annual average
Desert Southwest (Phoenix, Las Vegas, Albuquerque)	5.5–6.5
California, Colorado, Texas	4.5–5.5
Southeast (Florida, Georgia, Carolinas)	4.5–5
Midwest and Northeast	3.5–4.5
Pacific Northwest, upper Great Lakes	3–4

Two things stand out in that table. First, the spread is smaller than people assume. Seattle receives about 60% of the solar energy Phoenix does — a real difference, but nothing like the 5-to-1 ratio its reputation suggests. Germany, which for years led the world in installed solar capacity, sits at latitudes comparable to southern Alaska and averages fewer peak sun hours than any of the lower 48 states. Solar being better in Arizona doesn’t make it futile in Michigan.

Second, cloud climate matters as much as latitude. Miami sits far south of Denver but averages fewer peak sun hours, because Florida afternoons build clouds in exactly the hours that matter most, while Denver’s high, dry air is unusually transparent. Altitude helps too — thinner air absorbs less.

From Peak Sun Hours to Actual Electricity

The standard back-of-envelope estimate is:

Daily output (kWh) ≈ system size (kW) × peak sun hours × 0.8

The 0.8 is a derate factor covering everything between sunlight and your meter: inverter losses, wiring, dust and pollen on the glass, panel mismatch, and heat (more on that below). Industry modeling tools assume total system losses in the 14–20% range for a typical rooftop install, so 0.8 is a sensible round number — use 0.85 if the system is new and clean, less if panels face away from ideal or catch shade.

A worked example: a 7 kW rooftop system in a location averaging 5 peak sun hours produces roughly 7 × 5 × 0.8 = 28 kWh per day, or a little over 10,000 kWh per year — close to the annual consumption of an average US household. The same system in a 3.5-sun climate yields about 7,100 kWh. That single multiplication is, at its core, how solar quotes are built; everything else is refinement.

Heat Quietly Steals Output

Here’s the counterintuitive part: solar panels dislike heat. Standard silicon panels lose roughly 0.3–0.5% of output for every degree Celsius their cells run above the 25°C test condition — and cells on a sun-baked roof routinely run 25–30°C hotter than the surrounding air. On a 35°C Phoenix afternoon, cell temperatures can pass 60°C, shaving 10–15% off the panel’s rated output at the exact moment the sun is strongest.

This is why a crisp, clear April day in Colorado can out-produce a blazing July day in the low desert, watt for watt: full-strength sun on cold panels is the best case a panel ever sees. It’s also why peak sun hours alone slightly flatter hot climates and undersell cool, clear ones — the energy arriving is the same, but the panel converts more of it when it isn’t cooking.

Tilt and Direction: Getting the Geometry Right

Peak sun hour figures assume the surface is oriented sensibly. For a fixed panel in the US, that means facing south, tilted up from horizontal at roughly your latitude — the angle that best splits the difference between the high summer sun and the low winter sun. Adjusting seasonally (about latitude minus 15° in summer, latitude plus 15° in winter) recovers a few percent more; hardware that tracks the sun across the sky can add 25–35% in clear climates, though for rooftops the extra cost and maintenance rarely beat simply adding another panel.

East- or west-facing roofs give up something like 15–20% of annual energy compared to south — often a worthwhile trade. West-facing panels produce their peak in late afternoon, when household demand and time-of-use electricity rates are highest, so the value of a west-facing kilowatt-hour can exceed its energy shortfall.

Clear-Sky Numbers vs. Real Weather

One distinction to keep straight when comparing sources: some figures are clear-sky values — the energy a location would receive if no cloud ever passed — and some are weather-adjusted averages built from years of satellite and ground data. The city pages on this site show clear-sky peak sun hours computed from each city’s solar geometry, which makes them a clean upper bound and a fair way to compare the astronomy of two places: latitude, season, and sun angle, with the weather stripped out.

For a purchase decision, take the geometry as the ceiling and let local cloud climate discount it — in the desert Southwest reality lands close to the clear-sky number, in the Pacific Northwest well below it. The National Renewable Energy Laboratory’s free PVWatts calculator applies decades of measured weather to any US address and is the standard for turning peak sun hours into a bankable annual estimate.

Size for December, Not for the Average

One last habit worth borrowing from off-grid designers. Annual averages hide the seasonal swing, and the swing is large: a northern-tier city might see 6 peak sun hours in June and 1.5 in December. A grid-tied home can ride that out, banking summer surplus against winter deficit through net metering. An off-grid cabin cannot — its solar array must cover the worst month, not the average one, which is why off-grid systems in the north look so oversized in July.

Even for a grid-tied home, the monthly numbers are worth a glance before you sign anything. Every city page here lists clear-sky peak sun hours month by month alongside the noon sun altitude that drives them, and the winter rows tell you more about how a system will feel in February than the annual average ever will. The sun’s energy budget is written in geometry a year in advance; the only surprise left is the weather.