Washington · modelled at Seattle

Solar water pumps in Washington

Washington averages 3.93 peak sun hours a day across the year, 50th of 51. The darkest month, Dec, gives 1.32 (50th of 51 for winter sun); the brightest, Jul, gives 6.39. A pump that has to fill a trough in Dec needs 4.8 times the panels of one that only runs in Jul.

By the editors of SolarPoweredWaterPumps.comLast reviewed October 2026

Peak sun hours by month

1.5

Jan

2.7

Feb

3.6

Mar

5.1

Apr

5.8

May

5.9

Jun

6.4

Jul

5.8

Aug

4.5

Sep

2.8

Oct

1.8

Nov

1.3

Dec

kWh per square metre per day on a south-facing panel at 20 degrees in Seattle (47.61, -122.33). Source: NREL NSRDB via PVGIS v5.2.

Worked sizing for Washington

The same two systems are worked for every state so you can compare. Both use 35% or 30% wire-to-water efficiency and a 0.75 array factor; change any input in the calculator.

System	Design sun	Flow	TDH	Pump input	Panels	Tank
20 beef cattle, year-round (80 ft lift, 10 ft rise, 300 ft of 1 in poly)	1.32 h	5.05 gpm	97 ft	265 W	360 W	1,200 gal
Same well, grazing season April to October only	2.81 h	2.37 gpm	92 ft	117 W	160 W	1,200 gal
1,000 sq ft garden in summer, from a pond (8 ft suction, 15 ft rise, 200 ft of 3/4 in poly)	6.03 h	0.25 gpm	23 ft	4 W	10 W	267 gal

Panel figures are the floor from the arithmetic. Real systems usually carry 1.5 to 3 times this so the pump sits in the middle of its curve and the tank refills after cloudy days.

Run your own numbers. The sizing calculator shows every step, for your state, lift and pipe.

Open the calculator

What this means in Washington

Washington's winter sun is weak. A year-round direct-drive stock well works, but the array is large for the water it makes in Dec. Many owners size for the grazing season and haul or heat water in the darkest weeks, or add storage. Steeper winter tilt helps, and so does keeping panels clear of snow.

Related reading: winterizing, livestock watering and USDA cost share.

Sources