Running a water pump on solar or an inverter
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Water pumps are among the hardest loads for a small backup system. They draw a lot of power and surge when they start. There are two quite different ways to keep water flowing without the grid.
Option 1: run your existing pump from the backup inverter
This works if the inverter and battery are sized for it.
Starting surge. A pump’s motor can draw several times its running power for a moment as it starts. A 750 W (1 HP) pump might need over 2 kW briefly. The inverter’s surge rating, not just its continuous rating, must cover this on top of whatever else is running. See kVA vs kW.
Energy. Pumps usually run in short bursts, but they add up. A 750 W pump running 1 hour a day uses 0.75 kWh, as much as a fridge uses in a day.
Tips:
- Pump to a tank during the day, when solar is producing or the grid is on, rather than on demand at night.
- Avoid pressure pumps that cycle constantly on battery: every start costs energy and puts a surge on the inverter.
- Soft starters or variable-speed drives reduce the surge. Ask an installer whether your pump can use one.
- Use a pure sine wave inverter. Motors run hotter on modified sine wave. See pure vs modified sine.
Option 2: a dedicated solar pump and a storage tank
Solar pumps run directly from panels through a pump controller, usually with no battery at all. They pump while the sun shines and stop when it doesn’t. A raised tank stores the water, so you store water instead of electricity, which is far cheaper.
This suits:
- boreholes and wells that fill a tank used through the day
- farms, irrigation and livestock watering
- homes where the main backup system is too small to run the pump
Sizing a solar pump
Pump suppliers size these from:
- Daily water needed, in litres (or gallons) per day.
- Total head: how high the water must be lifted, from the water level in the well to the top of the tank, plus an allowance for pipe friction.
- Your sunshine: the dullest month’s peak sun hours where you live.
More water, a higher lift or less sun all mean a bigger pump and more panels. Give suppliers these three numbers and compare their pump performance charts, rather than comparing horsepower alone.
Which option to choose
| Situation | Better option |
|---|---|
| Small household pump, backup system already sized generously | Run it from the inverter, pumping to a tank |
| Borehole or well, water needed every day | Dedicated solar pump and tank |
| Farm or irrigation | Dedicated solar pump |
| Small backup system that struggles with surges | Dedicated solar pump, or a soft starter |
Add your pump to the calculator to see how it changes the inverter and battery you need.