Peak Sun Hours by US Region: Winter, Summer and Annual

Peak sun hours is the single input that decides whether a solar array carries your build or merely tops it up, and it varies by a factor of five across North America and by a factor of six across the seasons in one place. One peak sun hour means one hour of sunlight at 1,000 watts per square metre, which is the intensity panels are rated at. It is not a count of daylight hours. It is the whole day's varying intensity compressed into one number you can multiply a panel rating by.

What are the peak sun hours for each US region?

Every figure below is a range rather than a single number, and that is deliberate rather than evasive. Published regional averages differ between sources by half an hour or more depending on the tilt they assume and the years they average, and local conditions move the real figure far more than the difference between two neighbouring rows. Winter here means the worst months of the cold half of the year and summer means the best months of the warm half, so the two columns bracket what you will actually see rather than describing a specific date.

Three patterns run through that table. Dryness beats latitude: the Northern Plains at 48 degrees north do better in winter than the Gulf Coast at 30 degrees, because clear cold air passes more light than warm humid air. Elevation helps: the Colorado Plateau and the Rockies punch above their latitude for the same reason, and cold panels are efficient panels. And the two coasts are completely different problems: the Pacific coast loses to persistent low cloud through the cold half of the year, while the Atlantic side loses to summer haze and afternoon storms.

Say plainly what these numbers are. They are typical published regional averages for a fixed array at a modest tilt, and local terrain, shading and weather dominate them. A campsite under an oak canopy in the Texas Hill Country will not see 6.5 peak sun hours in any month. A canyon in southern Utah with walls on both sides loses the first and last two hours of the day regardless of what the regional figure says. A week of wildfire smoke can halve output across an entire state. Use this chart to size a system, then measure your own roof with a controller that logs daily yield and trust that instead.

How big is the seasonal swing, and why does it decide the array?

Because you cannot average your way out of a dark month. A battery buffers a few days, not a season. If your array delivers 40 percent of the load through the cold half of the year, the system does not deliver 100 percent on average and 40 percent in winter: it delivers 100 percent in summer and fails every single day in winter until something else charges the battery.

The table below takes the low end of each winter range and the high end of each summer range, which is the honest worst case, and shows what a 400W array delivers at each.

A typical full-time build uses around 105 amp-hours a day. Read down the winter column and count how many regions a 400W array covers that in the cold months. Exactly one clears it outright: Baja, at 113. The Southwest desert and Florida come within five amp-hours at 100 each, which is close enough to work if you are careful. Texas manages 75. Everywhere else the array falls short in winter by somewhere between a quarter and effectively everything, and the Pacific Northwest coast delivers 25 amp-hours against a 105 amp-hour load. Now read the summer column, where every single region except coastal Alaska is comfortable.

That gap is the single most important thing on this page, and it explains a pattern in how people talk about their builds. Somebody with 400W who spends summers in Colorado will tell you solar runs their whole van, and they are telling the truth about their situation. The same rig in Washington in the cold months would not run the fridge. Neither person is wrong, they are quoting different columns.

Two honest ways out. Add array, which works until the roof runs out, or add a charging source that does not care about the sky. A Renogy Smart 50A DC-DC MPPT Battery Charger 12V (Dual Input) delivers roughly 50 amp-hours for every hour of driving, so two hours on the road replaces more than a 400W array does through an entire grey winter day. That is why the solar versus alternator comparison concludes that they are partners rather than alternatives, and why the winter camping power guide leans on the alternator so heavily.

How much array do I need at each sun hour figure?

Work it backwards. Required array watts equals daily amp-hours times 12, divided by peak sun hours, divided by the 0.75 derate. The table runs that for three realistic daily loads: 50 amp-hours for a weekend build with a fridge and lights, 105 for a typical full-time van, and 205 for a build that cooks on electricity. Results are rounded up to the nearest 25W.

Look at where the table becomes absurd. At 5 peak sun hours a full-time build needs 350W, which fits comfortably on a mid-length van roof as two 200W panels. At 2 peak sun hours it needs 850W, which does not fit on most vehicles and would not be worth the money if it did. At 1 peak sun hour it needs 1,700W, which is a small residential array bolted to a Sprinter.

That is the practical reason nobody solves a northern winter with solar. The correct response below about 2.5 peak sun hours is not a bigger array, it is a different charging source and more battery to ride through. Above about 4 peak sun hours, solar is the cheapest energy in the vehicle by a wide margin and worth maximising. The solar array calculator runs this arithmetic on your own load, and the solar output by sun hours chart shows the yield side of it in full.

What moves your real figure away from the regional average?

Shading, first and by a long way. Cells in a panel are wired in series, so a shadow across one corner throttles the whole panel rather than the fraction that is dark. On a vehicle the shadows are usually your own: a roof fan, an antenna, a rack crossbar, a light bar, a surfboard. Then they are the campsite's: trees, canyon walls, the side of a building, another vehicle. A rig that parks in the shade for comfort in Arizona in high summer will do worse than the regional figure even though the sky is perfect.

Tilt and aim. A flat roof panel is optimised for one moment near solar noon and compromised at every other hour, and the loss is worst exactly when the sun is lowest, which is winter at high latitude. This is where a portable panel earns its price: park in the shade, put the panel in the sun, and aim it. A folding 100W panel plugged in through an SB50 Anderson connector can out-produce 150W of flat roof panel on a low winter sun day, and the portable versus roof-mounted comparison covers the trade in full.

Weather patterns that averages hide. Two regions with the same annual figure can behave completely differently: one delivers 4.5 peak sun hours most days, and the other alternates between 7 and 0.5 in multi-day blocks. The second needs far more battery for the same array, because the battery has to carry three consecutive dark days rather than shave the edges off a consistent one. The Sierra Nevada and the Northeast in the cold months are both like this, and a 200Ah self-heating pack is a more useful purchase there than another 200W of panel.

Temperature, in both directions. Panels lose roughly 0.35 to 0.45 percent of their output per degree C above 25C, so a Southwest summer at 6.5 peak sun hours does not deliver 6.5 sun hours worth of energy: the panels are running 30C above ambient and giving up 15 percent or more. Conversely a clear cold day in the Rockies gives you slightly more than the sun hours imply. This is one reason the Southwest and the Rockies are closer in real yield than the raw sun hour figures suggest.

And snow, which is binary rather than gradual. A few centimetres on a flat roof array stops production almost completely, and a flat panel has no angle to shed it. In a northern winter build this is the strongest argument for tilt brackets, though a long-handled soft brush kept inside the vehicle solves it faster and cheaper. The solar panel mounting guide covers both.

How should I actually use this chart?

Pick the region you spend the most nights in, not the one you dream about. Then pick the column for the worst season you will genuinely be there in. If you are a summer traveller, use the summer column and enjoy the smaller array. If you live in the vehicle year round and follow the weather south, use the winter column of wherever you actually go, which for many people is Arizona or Baja rather than the state on their registration.

Then add margin for the fact that this chart is regional and your roof is specific. A reasonable convention is to size from the worst-month figure and then check the result against the best-month figure to make sure you have not bought an array that will spend six months clipping at the controller. Oversized arrays are not wasted: an array that only reaches full output for two hours in winter is still capturing energy at the shoulders of the day when a smaller one would produce nothing usable at all.

Finally, measure. Every serious build ends up with a shunt and a controller that logs, and the real yield figure for your own roof over a real month is worth more than any published average including this one. Everything here is drawn from published regional averages and standard solar derate conventions rather than measured by us. It is researched guidance rather than an electrical certification, and it does not replace ABYC E-11 or the manuals supplied with your panels and controller. Fuse the controller output at the battery within a few inches of the terminal, size cable for voltage drop as well as ampacity, and have any lithium installation inspected by a qualified installer before it carries load.

Where to go next

• Solar panel output by sun hours, the yield table these figures feed.
• Solar array calculator, your load and your region together.
• Solar versus alternator charging, what to do about the winter column.
• Winter camping power guide, running a build where solar cannot.
• Best solar panels for a van, the panels themselves compared.