Peak sun hours in Washington
Washington averages 3.8 peak sun hours a day across the year, 49th of the 50 states. At that figure a 400-watt panel produces about 1.17 kWh a day after real-world losses, and covering the 900 kWh average US bill takes about 26 panels. Seattle, the state's biggest city, sits a little under the statewide figure at 3.6 hours, so 3.8 is the midpoint of a spread that leans away from the largest population centre.
cloudy-country sun: 3.8 of a possible ~6.5 hours
Panel counts for Washington bills
Computed at Washington's 3.8 hours with 400W panels and the 0.77 derate; the US-average column shows what the same bill takes at 4.5 hours, so you can see what your state's sun is worth in hardware.
| Monthly usage | Washington | System size | US average |
|---|---|---|---|
| 500 kWh/mo | 15 panels | 6 kW | 12 panels |
| 750 kWh/mo | 22 panels | 8.8 kW | 18 panels |
| 900 kWh/mo | 26 panels | 10.4 kW | 22 panels |
| 1,000 kWh/mo | 29 panels | 11.6 kW | 24 panels |
| 1,500 kWh/mo | 43 panels | 17.2 kW | 36 panels |
| 2,000 kWh/mo | 57 panels | 22.8 kW | 48 panels |
What 3.8 hours means in practice
Washington is at the cloudy end of the table: 49th of 50 at 3.8 peak sun hours, 16% under the national average. Covering the 900 kWh average bill takes 26 panels here against 22 nationally. Solar still works at this latitude and cloud cover, and panels run cooler here, which claws a little of it back.
How a Washington year is shaped
The 3.8-hour figure is an annual average, and no month is average. These are the twelve monthly means for Washington from NASA's 2001-2020 solar climatology, each as a share of the state's own annual mean, with that share applied to the 3.8-hour planning figure. July and December are marked.
| Month | Share of the annual average | Peak sun hours (est.) |
|---|---|---|
| January | 30% | 1.1 |
| February | 56% | 2.1 |
| March | 84% | 3.2 |
| April | 123% | 4.7 |
| May | 153% | 5.8 |
| June | 168% | 6.4 |
| July | 187% | 7.1 |
| August | 157% | 6.0 |
| September | 114% | 4.3 |
| October | 68% | 2.6 |
| November | 36% | 1.3 |
| December | 24% | 0.9 |
Washington swings harder between seasons than any state except Alaska: 7.8 to one from July down to December, with December returning 24% of the annual average. Four months carry 55% of the year. The summer half is genuinely strong, about 7.1 hours a day at the July peak, which beats Georgia's annual figure outright. The winter half is why people write the whole state off, and that write-off is wrong for roughly eight months of it.
Half the Washington calendar sits above the annual average and half below: April through September are the six months that beat it, carrying 75% of the year's light. April is already worth about 4.7 hours a day, so the productive half of the year starts earlier than most people expect.
Bills here run above the national line. The average Washington household used 956 kWh a month in 2024 against 863 nationally, 11% more, and covering that across a year takes 27 panels at Washington's sun. Covering it from December sun alone would take 113, 4.2 times the array, which is the number that decides whether a battery plan is realistic.
Washington is one of the states where a statewide number does the most damage: it has the 8th widest internal spread of the fifty. Across the 20 grid cells sampled here the southeast runs 23% sunnier than the west. Held against the 3.8-hour statewide figure that is roughly 4.2 hours at one end and 3.4 at the other, which is the difference between two quite different quotes. Nobody should size a Washington roof off the state average without checking which end of the state it sits on.
What that sun is worth on a Washington bill
| EIA, 2024 residential averages | Washington | United States |
|---|---|---|
| Average home's monthly use | 956 kWh | 863 kWh |
| Price per kilowatt-hour | 11.9¢ | 16.48¢ |
| Average monthly bill | $113.68 | $142.26 |
| One 400W panel, a year's output | 427 kWh | 506 kWh |
| What that output displaces | $51 | $83 |
Washington has some of the cheapest electricity in the United States, 11.9¢ a kilowatt-hour in 2024 against a national 16.48¢, 5th cheapest of the fifty. That is 28% less value in every kilowatt-hour a panel produces: about $51 a year from one 400-watt panel where the national figure is $83. Sun is not this state's constraint on solar. The rate it displaces is.
Washington lands in much the same place on both rankings, 49th for sun and 50th for what a panel earns, so its position in the sun table is a fair guide to its position on value. That is not automatic. New York gets more sun (3.9 hours to Washington's 3.8) and pays 24.43¢ against 11.9¢, so one panel is worth about $56 less a year in Washington than in New York.
Two warnings on those dollar figures. They are displaced electricity and nothing else: no install cost, no incentive, no financing, and no assumption about how your utility settles a surplus, which is the single biggest unknown in the sum. And a state average hides the utility you are actually on; 11.9¢ is every Washington residential customer averaged together, and neighbouring territories can differ by more than the gap between two states in the ranking.
Getting a number for your address
For a location-exact figure, run your address through NREL's free PVWatts; it uses measured weather for your grid cell and will also model roof tilt and direction, which statewide numbers cannot. Then compare its annual kWh against a quote's promise before you sign anything.
Questions people ask
How many solar panels do I need in Washington?
About 26 400-watt panels (10.4 kW) for the 900 kWh a month an average US home uses, at Washington's 3.8 peak sun hours. For the average Washington home specifically, which EIA measured at 956 kWh a month in 2024, it is 27. Your own bill is the variable that matters, and the calculator takes any figure.
Is 3.8 peak sun hours good for solar?
It is below the 4.5-hour national average but far from disqualifying. Panel counts run about 18% higher here for the same bill, and the December trough measured above deserves respect in any off-grid plan. Grid-tied systems ride through it on net metering.
How much is one solar panel worth a year in Washington?
About $51. One 400-watt panel at Washington's 3.8 peak sun hours makes roughly 427 kWh a year after the 0.77 derate, and Washington residential electricity averaged 11.9¢ a kilowatt-hour in 2024 (EIA), which puts Washington 50th of the 50 states on this measure against 49th on sunlight alone. That figure is displaced electricity only. It says nothing about install cost, incentives, or how your utility settles a surplus.
Do peak sun hours change with the seasons in Washington?
Yes, by a factor of 7.8 here. NASA's 2001-2020 climatology puts Washington's July at 187% of the annual average and December at 24%, so the 3.8-hour figure on this page is the midpoint of a curve running from roughly 7.1 hours a day down to 0.9. Grid-tied homes bank the summer surplus as credit; off-grid designs have to size for the trough, and in Washington that means 113 panels for an average local bill instead of 27.