Off-Grid Solar Calculators

Appliance-specific

Off-Grid Well Pump Solar Sizer

A ½ HP 240 V pump might run near 1,000 W (about 4 A FLA) but locked-rotor amps are often 5–7× that. Energy is running watts × hours plus the inverter loss; the inverter must still cover the surge.

Inputs

Units
W

≈ watts = FLA × voltage. A ½ HP 240 V motor is often near 1,000 W running.

h
h

Usable fraction of rated capacity.

System voltage of the battery or series string.

The load is AC, so the battery and array carry the conversion loss.

Terms of Service

Results

Please tick the box above to display the Result.

Engineering methodology

Daily watt-hours = running watts × pump hours, divided by inverter efficiency because the bank feeds an AC motor. Surge watts = running watts × LRA factor (often 5–7× for induction motors) and stay on the AC side. Battery amp-hours = daily Wh ÷ (V × DoD). Array watts = daily Wh ÷ (peak sun hours × derate). When you can, compute running watts from nameplate FLA × voltage (P = V × I); on an AC motor that product is volt-amperes, slightly above real watts, so the result errs on the safe side.

Frequently asked questions

What LRA surge factor should I use for a well pump inverter?

Read locked-rotor amps from the motor nameplate. LRA / FLA is often about 5–7× without a soft starter. Soft-start kits can cut that toward 2–3×, though 3× is the lowest factor this calculator offers. Surge is for the inverter, not extra daily watt-hours.

How many watts is a ½ horsepower well pump?

Mechanical ½ hp is about 373 W, but electrical input is higher. A 240 V ½ hp motor might run near 4 A FLA, about 1,000 W. Always use FLA × voltage from the nameplate.

Does a pressure tank reduce solar and battery size?

It reduces how often the pump starts and can shorten total run hours, which lowers daily Wh. It does not remove the surge the inverter must still supply at each start.

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