Washington State Solar Panel Performance Benchmarks
Last updated: 2026-04-08 · Solar Benchmark
Washington State Solar Panel Performance Benchmarks
A 6kW solar system in Washington State produces between 5,880 and 9,240 kWh per year depending on location. The range is unusually wide because the Cascade Range creates two entirely different climates. Seattle and western Washington average about 6,240 kWh for a 6kW system. Eastern Washington cities such as Spokane (8,520 kWh), Yakima (9,000 kWh), and the Tri-Cities near Kennewick (9,240 kWh) rival production levels found in much sunnier states.
Monthly Production Benchmarks: Washington State 6kW Reference System
Expected monthly production for a south-facing, 30-degree tilt, 6kW system. Derived from pvlib simulation using Open-Meteo ERA5 historical weather data, Seattle as the western Washington reference location.
| Month | Expected Production (kWh) | Notes |
|---|---|---|
| January | 140 | Very low sun, persistent cloud |
| February | 240 | Marginal improvement |
| March | 440 | Spring ramp begins |
| April | 620 | Meaningful shoulder production |
| May | 790 | Near-peak, long days |
| June | 800 | Peak production, long days |
| July | 1,010 | Best month: clear and long |
| August | 960 | Remains strong |
| September | 700 | Fall transition |
| October | 400 | Rapid decline |
| November | 220 | Near-winter lows |
| December | 140 | Lowest month, very little sun |
| Annual Total | ~6,460 | Seattle-area 6kW reference |
(Source: pvlib physics modeling, Open-Meteo ERA5 weather data)
Annual Benchmarks by System Size and Washington State Region
| System Size | Seattle / Puget Sound | Bellingham | Olympia | Spokane | Yakima | Tri-Cities (Kennewick) |
|---|---|---|---|---|---|---|
| 4 kW | 4,160 | 3,880 | 3,920 | 5,680 | 6,000 | 6,160 |
| 6 kW | 6,240 | 5,820 | 5,880 | 8,520 | 9,000 | 9,240 |
| 8 kW | 8,320 | 7,760 | 7,840 | 11,360 | 12,000 | 12,320 |
| 10 kW | 10,400 | 9,700 | 9,800 | 14,200 | 15,000 | 15,400 |
| 12 kW | 12,480 | 11,640 | 11,760 | 17,040 | 18,000 | 18,480 |
Seattle specific yield: ~1,040 kWh/kW/year. Bellingham: ~970. Olympia: ~980. Spokane: ~1,420. Yakima: ~1,500. Tri-Cities: ~1,540.
(Source: pvlib physics modeling, Open-Meteo ERA5 weather data, 2015–2024 averages)
Washington State Climate Zones and Performance Ratio Targets
Washington State has one of the largest intrastate solar production differences in the country. A Tri-Cities system produces nearly 50% more annual energy than a Seattle system of the same size. Performance ratios below 0.76 in any Washington region warrant investigation.
| Climate Zone | Representative Area | Annual Specific Yield | Expected PR Range |
|---|---|---|---|
| Western WA (Marine) | Seattle, Bellingham, Olympia, Tacoma | 960–1,060 kWh/kW | 0.76–0.84 |
| Eastern WA / Inland Valleys | Spokane, Pullman, Walla Walla | 1,390–1,450 kWh/kW | 0.78–0.86 |
| Central WA (Yakima Valley) | Yakima, Ellensburg, Wenatchee | 1,460–1,530 kWh/kW | 0.79–0.87 |
| Columbia Basin | Kennewick, Richland, Pasco | 1,510–1,570 kWh/kW | 0.80–0.88 |
Learn more about how these benchmarks are calculated at /resources/methodology.
What Affects Washington State Solar Output
- The Cascade Range effect: The Cascades force marine air upward as it moves inland from the Pacific, wringing out moisture on the western slopes and creating a rain shadow to the east. Seattle sits in the marine zone and receives frequent cloud cover and drizzle for nine months of the year. Spokane, just 280 miles east, gets approximately 40% more annual solar irradiance. This is not a system or installation issue, it is geography.
- Seattle's dramatic seasonal swing: July is Seattle's best solar month by a wide margin, producing about 1,010 kWh for a 6kW system. January produces only 140 kWh. The 7-to-1 peak-to-trough ratio is the largest seasonal swing of any major US city. This swing is normal and expected. A Seattle system producing 400–500 kWh in January is not underperforming.
- Eastern Washington summer irradiance: The Columbia Basin and Yakima Valley receive summer irradiance that rivals Arizona. Tri-Cities July solar radiation exceeds Tucson in some years due to low humidity and clear skies. Eastern WA systems accumulate the bulk of their annual production between April and September.
- Snow soiling patterns: Western Washington rarely sees persistent snow at low elevations. Seattle panels typically self-clean within hours of any snowfall. Eastern Washington does see snow accumulation from November through February, and panels in the Spokane area benefit from monitoring during cold snaps.
- Net metering in Washington: Washington State has relatively modest retail electricity rates compared to California or New England. Net metering rules and the overall financial return are less dramatic than in high-rate states, but the Cascade factor means eastern WA systems generate significantly more energy per dollar of installation cost than western WA systems.
Frequently Asked Questions
Q: What should a 6kW solar system produce per month in Seattle?
A: The Seattle benchmark averages about 538 kWh/month, but the distribution is highly uneven. July peaks at roughly 1,010 kWh; December and January drop to 140 kWh each. Consistent summer production below 800 kWh/month (July-August) on a 6kW Seattle system suggests a problem worth investigating. Winter lows in the 130–160 kWh range are normal.
Q: Is solar worth installing in Seattle given the cloud cover?
A: Seattle receives about 6,240 kWh/year from a 6kW system, which is less than Phoenix but comparable to Portland and better than some northern European cities with very high solar adoption. The financial case depends heavily on local electricity rates and available incentives. Eastern Washington locations are dramatically more productive and present a stronger energy-volume case.
Q: How much more does an eastern Washington system produce than a Seattle system?
A: A 6kW system in the Tri-Cities (Kennewick) produces about 9,240 kWh/year versus 6,240 kWh/year in Seattle. The 48% difference is entirely due to climate. Eastern WA systems also benefit from hot, low-humidity summers that push irradiance to very high levels in July and August.
Q: How do I get an independent benchmark for my Washington State solar system?
A: A valid benchmark requires production data and actual hourly weather at your address. Western and eastern Washington are so climatically different that a statewide average is meaningless for individual system diagnosis. ERA5 weather data resolves this by providing location-specific historical irradiance. Learn more at /resources/methodology.
Data: pvlib physics modeling + Open-Meteo ERA5 weather data | Last updated: 2026-04-08 | Solar Benchmark