What is a peak sun hour?
A peak sun hour is 60 minutes during which sunlight hits 1,000 watts per square meter (1 kW/m2), which is the reference intensity panels are tested and rated at. Real sunlight is almost never at that level all day; it builds from nothing at dawn, climbs to a midday peak, and fades to nothing at dusk. Peak sun hours take all of that up-and-down light and add it up into an equivalent count of full-strength hours. A day that produced weak morning sun, strong midday sun, and weak evening sun might add up to 5 peak sun hours even though the sun was above the horizon for 13.
The number is really a measure of energy, not time. One peak sun hour equals 1 kilowatt-hour of sunlight energy per square meter, so your peak sun hours are the same as your daily solar energy total (measured in kWh/m2 per day). If a solar map says your area receives 4.5 kWh/m2 per day, that is 4.5 peak sun hours. This is why the term shows up everywhere in solar sizing: it is the one figure that converts sunshine into an amount of power you can plan around.
Are peak sun hours the same as daylight hours?
No, and mixing the two up is the most common mistake. A summer day might give you 14 hours of daylight but only 6 peak sun hours, because the light near sunrise and sunset is far below 1,000 watts per square meter and barely counts. Panels still make a little power in that weak light, but it takes several hours of it to add up to one full peak sun hour. The count only reflects the strong, productive part of the day.
Think of it like filling a bucket with a hose whose pressure changes all day. Daylight hours tell you how long the hose ran; peak sun hours tell you how much water actually came out. That is why a place with long summer days can still have modest peak sun hours if it is often hazy or overcast, and why a short, clear winter day in the desert can outproduce a long, gray one somewhere cloudy.
How many peak sun hours does my area get?
Most of the United States averages 3 to 6 peak sun hours per day over the year. The desert Southwest (Arizona, Nevada, New Mexico) leads at about 6 to 7, the middle of the country sits near 4.5 to 5.5, and the Pacific Northwest and Northeast run about 3.5 to 4.5. Seattle averages close to 3.5, while Phoenix is near 6.5. That spread is large enough that the same house needs noticeably more panels in a cloudy region than in a sunny one, which our guide on how many solar panels to power a house walks through.
These are annual averages, and they hide a big seasonal swing. A northern state might see 6 peak sun hours in June and under 2 in December, so winter production can be less than half of summer, as do solar panels work in winter explains. When you size a system you generally use the yearly average, then lean on net metering or a battery to smooth the seasons out.
To find your own figure, use a free tool rather than a rough map. NREL's PVWatts calculator lets you type in your address and returns the solar resource for your exact location and roof tilt; the Global Solar Atlas and NREL's National Solar Radiation Database do the same. Any of them will give you the daily kWh/m2 value, which is your peak sun hours. Once you have it, drop it into the solar panel calculator to turn it into a panel count for your usage.
How do you calculate solar output from peak sun hours?
The core formula is short: daily output = system watts x peak sun hours x about 0.8. The 0.8 is a real-world derate that accounts for the energy lost to heat, inverter conversion, wiring, and dust, which together trim roughly 15 to 25 percent off the lab rating. So a single 400-watt panel in an area with 5 peak sun hours makes about 400 x 5 x 0.8 = 1,600 watt-hours, or 1.6 kWh a day. Scale that up: a 6,000-watt system in the same spot makes about 24 kWh a day.
This is exactly why peak sun hours matter more than panel wattage on its own. The same 400-watt panel makes close to 1.9 kWh a day in Arizona but nearer 1.1 kWh in Seattle, purely because of the difference in peak sun hours. It is also why a panel almost never hits its rated wattage in practice; that rating assumes a full 1,000 watts per square meter and a cool 25C at the same time, conditions that rarely line up. Our guide on how much energy a solar panel produces breaks the output math down further.
For anything you plan to buy, do not stop at the average. Your roof tilt, orientation, shading, and local weather all move the result, so run your real numbers through the solar panel calculator instead of trusting a single national figure.
What changes your peak sun hours?
Four things move the number: season, weather, tilt, and shade. Season is the biggest. The sun sits low in winter, so its light travels through more atmosphere and spreads across a wider area, cutting winter peak sun hours to as little as half of the summer figure in northern states. Weather stacks on top of that; a heavy overcast can drop a day to 10 to 25 percent of clear-sky output, though panels keep working on cloudy days because they run on the diffuse light that filters through.
Tilt and orientation decide how much of the available sun the panels actually catch. In the Northern Hemisphere, panels facing true south and tilted at roughly your latitude capture the most over a year. A flat or north-facing array, or one steeper or shallower than ideal, sees fewer effective peak sun hours even under the same sky. This is a fixed choice you make at install, so it is worth getting right.
Shade is the quiet killer. Because panels in a string share their weakest link, a tree branch or chimney shadow crossing even one panel for part of the day can pull down more than its share of production. Good installers map the shade across the seasons before they place panels, and hardware like microinverters or optimizers limits the damage when some shade is unavoidable.
Why do peak sun hours matter for sizing a system?
Peak sun hours are the bridge between your electric bill and the size of the system you need. You start with how many kWh you use in a day, divide by your peak sun hours and the 0.8 derate, and you get the system wattage that would cover it. A home using 30 kWh a day in a 5-peak-sun-hour region needs roughly 30 / (5 x 0.8) = 7.5 kW of panels; the same home in a 3.5-hour region needs closer to 10.7 kW. Same house, very different array, entirely because of peak sun hours.
That is the single reason two identical homes get very different solar quotes in different states. Rather than work the math by hand, put your usage and location into the solar panel calculator, which handles the peak sun hours and derate for you, and check the result against real pricing in how much solar costs.
Frequently asked questions
How do I find my peak sun hours?
Use a free solar resource tool. NREL's PVWatts calculator returns the value for your exact address and roof tilt, and the Global Solar Atlas and NREL's National Solar Radiation Database do the same. Each one gives you the daily solar energy in kWh per square meter, and that number is your peak sun hours. A peak-sun-hour map of your state gives a quick average, but a tool that uses your address is more accurate because it accounts for your local climate.
What is meant by peak sun hours?
A peak sun hour is one hour of sunlight at an intensity of 1,000 watts per square meter, the level panels are rated at. Because real sunlight is weaker in the morning and evening, peak sun hours condense a full day of changing light into an equivalent number of full-strength hours. One peak sun hour equals one kilowatt-hour of sunlight energy per square meter, so the figure doubles as your daily solar energy total.
Is 2 pm peak sun?
Not quite. Sunlight is strongest at solar noon, when the sun sits highest in the sky, which usually falls closer to 12 to 1 pm and shifts a bit with daylight saving time and where you are within your time zone. By 2 pm the sun has started to drop, so intensity is a little past its peak, though 2 pm is still inside the strong production window of roughly 10 am to 4 pm. Keep in mind that peak sun hours are a daily energy total, not a specific time of day.
What is the 20% rule for solar?
It is a rule of thumb that a system's real output runs about 20 percent below its nameplate rating, because heat, inverter conversion, wiring, and dust all shave off some energy. In practice the losses land somewhere around 15 to 25 percent, which is why the standard output math multiplies watts times peak sun hours by roughly 0.8. It is a planning estimate, not a fixed law; your actual derate depends on your climate, wiring, and how clean the panels stay.
How many peak sun hours do you need for solar to be worth it?
There is no hard minimum. Solar works across the entire US, including cloudy regions like the Pacific Northwest, which average about 3.5 peak sun hours. Fewer peak sun hours simply means you need more panels to make the same energy, so a low-sun area can still pay off when electricity is expensive. Areas with 4 or more peak sun hours are comfortably solar-friendly, but your electric rate and roof usually matter more than sun alone.