Reference

Peak Sun Hours by State

A peak sun hour is one hour of full midday sun, the standard 1000 watts per square meter that solar panels are rated against. It is not the same as an hour of daylight. The number of peak sun hours your location gets is the single figure that decides how much panel you need, because it tells you how many "rated" hours of charging you can count on in an average day.

Use the chart below to find your state, then jump to how to turn that number into a panel count. The annual figure is the daily average across a whole year. The winter and summer columns show how far the real number swings, which matters more than most beginners expect.

Peak sun hours by state (chart)

Figures are state-average daily peak sun hours grounded in NREL National Solar Radiation Database data. They are approximate, vary by location and tilt within each state, and are lower in cloudy microclimates. For a site-specific number, run your exact address through NREL PVWatts.

What is a peak sun hour?

A peak sun hour is one hour during which sunlight hits a surface at an intensity of 1000 watts per square meter, which is the same "full sun" intensity used to rate every solar panel. So if your location gets 5 peak sun hours, it means the total solar energy that landed there over the whole day equals 5 hours of that ideal full-strength sun.

That is why a 100 watt panel in a 5 peak sun hour location produces roughly 100 watts times 5 hours, or about 500 watt-hours, on an average day before losses. The peak sun hour figure does the hard work of turning a messy, changing sky into one clean number you can multiply against panel wattage.

How to use peak sun hours to size your solar

Sizing a solar array is short arithmetic once you know your sun hours. Start with the energy you use in a day, in watt-hours, then work backward to the panel wattage that refills it.

The core formula is:

The 1.25 is a loss factor. Charge controllers, wiring, heat, dust, and battery round-trip losses mean a panel never delivers its full rating into the battery, so you add about 25 percent of headroom.

Worked example. Say a small cabin uses 1,500 watt-hours a day for a fridge, lights, a laptop, and phone charging. In a state averaging 5 peak sun hours:

• 1,500 Wh ÷ 5 peak sun hours = 300 watts of "ideal" panel
• 300 W × 1.25 loss factor ≈ 375 watts of real panel
• So you would install around 400 watts of panel, for example two 200 watt panels

Now run the same cabin in a 4 peak sun hour state: 1,500 ÷ 4 × 1.25 is about 470 watts, so you would step up to roughly 500 watts of panel for the same daily energy. Fewer sun hours simply means more panel. Our solar calculator and battery bank calculator do this math for you, and how to size an off-grid solar system walks through a full build step by step.

Why winter sun hours matter most

The annual average is a comfortable number, but it can fool you. Look at the winter column in the chart: in much of the country, December and January deliver only about half the annual figure. The sun sits low, days are short, and that is also when storms park overhead for days at a time.

If solar is your only power source and you live there year-round, you must size for your worst month, not the yearly average. A system that sails through July can starve in January. A cabin in a 4.2 annual sun-hour state might really be planning around a 2.2 winter figure, which roughly doubles the panel and battery you need compared to a summer-only estimate.

If you only use the place in the warm months, the summer column is your guide and you can build smaller. Be honest about how you will actually use the system. Our guide on how many solar panels to power a cabin works through both seasonal cases.

Peak sun hours vs hours of daylight

This is the confusion that wrecks more first-time solar plans than any other. Hours of daylight is just the time the sun is up, which can be fifteen hours on a long June day. Peak sun hours is a measure of energy, and it is always a smaller number.

Early-morning and late-evening sun comes in at a shallow angle through more atmosphere, so it is weak. Only the hours around midday approach that full 1000 watts per square meter. Add it all up and even a long summer day usually nets six to eight peak sun hours, while a short, overcast winter day might net only one or two despite nine hours of "daylight."

The rule is simple: always size a solar system with peak sun hours, never with daylight hours. If you size by daylight, you will badly undersize the panels and wonder why the battery never fills. To see how all the pieces connect, start with off-grid solar explained and our solar panels guide.

Pick the right panel for your sun hours

In a high sun-hour state you can hit your daily target with less panel, so a couple of efficient rigid panels often do the job. In a low sun-hour or winter-heavy location, you want more wattage and, for flexibility, a portable panel you can angle and move to chase the sun. For a cabin specifically, our best solar panels for a cabin roundup covers the rigid options worth buying.

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Related references and tools

• Appliance power consumption chart, to total your daily watt-hours
• Solar calculator and battery bank calculator
• How to size an off-grid solar system
• How many solar panels to power a cabin
• Solar panels guide