Short answerWork out the pump's daily energy (running watts x hours a day), divide by your peak sun hours and by about 0.75 for losses, and add about 10% more if a battery sits in between. A 1,000 W pump running one hour a day needs about 300 W of panels with 5 sun hours, plus an inverter that can start it.
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Key takeaways
- DC solar pumps are sized from water and lift; existing AC pumps from daily energy.
- Panels = daily Wh / sun hours / 0.75, and / 0.9 more through a battery.
- For an AC pump the inverter's surge rating, not the panels, is the hard part.
If you are choosing a DC solar pump
Start from water, not from panels. Daily gallons and total dynamic head give the pump's power, and the pump's power gives the panels. The watts by well depth table does that for common cases, and the calculator does it for yours. A direct-drive pump also needs enough panel voltage to start: see panel wiring and voltage.
If you are keeping an AC pump
An AC well pump draws its full running power whenever it runs, then stops. The panels only have to replace the energy it used, through a battery. Measure the running watts with a clamp meter at the pump circuit, or multiply the motor's full-load amps by its voltage, then estimate the hours it runs in a day (gallons used / pump gpm / 60).
| Pump running watts | 0.5 h a day | 1 h a day | 2 h a day | 3 h a day |
|---|---|---|---|---|
| 500 W | 0.25 kWh / 100 W | 0.50 kWh / 150 W | 1.00 kWh / 300 W | 1.50 kWh / 450 W |
| 750 W | 0.38 kWh / 150 W | 0.75 kWh / 250 W | 1.50 kWh / 450 W | 2.25 kWh / 700 W |
| 1,000 W | 0.50 kWh / 150 W | 1.00 kWh / 300 W | 2.00 kWh / 600 W | 3.00 kWh / 900 W |
| 1,500 W | 0.75 kWh / 250 W | 1.50 kWh / 450 W | 3.00 kWh / 900 W | 4.50 kWh / 1,350 W |
| 2,000 W | 1.00 kWh / 300 W | 2.00 kWh / 600 W | 4.00 kWh / 1,200 W | 6.00 kWh / 1,800 W |
Worked example
A household uses 200 gallons a day from a 10 gpm submersible: 20 minutes of running. If the pump draws 1,000 W, that is 0.33 kWh a day. Divided by 5 sun hours, 0.75 for panel losses and 0.9 for the battery round trip, it needs about 100 W of panels. The panels are cheap here; the inverter that can start the motor and the battery behind it are the real cost. See running an AC well pump on solar.
Rules of thumb that hold up
- Size for the worst month you need water in, not the annual average. Our state pages show both.
- Add margin for panel ageing. NRCS's pumping plant standard asks designers to allow for at least 10 years of panel degradation at no more than 1% a year.
- Count panels in whole strings. A 24V-class pump wants panels in series pairs, so 300 W of need becomes four 100 W panels, not three.
Questions
- How many 100 W panels to run a 1/2 hp well pump?
- It depends on how long the pump runs, not its horsepower alone. A pump drawing about 1,000 W for one hour a day uses 1 kWh, which needs about three 100 W panels with 5 sun hours through a battery. It also needs an inverter rated for the motor's starting surge.
- Can I run a well pump directly from solar panels without a battery?
- Only a pump built for it: a DC solar pump with its own controller, or an AC pump on a solar pump inverter. A standard AC well pump needs steady voltage and a large starting current, which panels alone cannot supply.
Put your own gallons, lift and pipe in and see every step of the working.