How much power a 12V water pump really uses

A pump label gives you a couple of numbers, and those numbers decide whether a small battery will cope. The sums are short. Once you've done them, you'll know before you buy.

Volts, amps and watts

Volts are the electrical push. Amps are how much current flows. Watts are the power the pump uses at that moment. A university guide gives the link: watts equal volts times amps¹. The same guide calls voltage range, watts and maximum current the pump specifications that matter most¹.

Here's a made-up example, only to show the sum. Say a label reads 12 volts and 5 amps. Multiply them and you get 60 watts¹. Your pump's label will have its own figures.

Look for the maximum current on the label, and check your maker's manual for what each figure means.

Watt-hours: power over time

Watts tell you the draw while the pump runs. They don't tell you how much a day's pumping takes. For that you want watt-hours, written Wh.

One watt-hour is what a one watt load uses in an hour. The larger kilowatt-hour is the energy that one kilowatt delivers in an hour². The same page gives a comparison: a 40-watt appliance left on for 25 hours uses as much energy as a one kilowatt heater does in one hour².

For a pump, energy is the power requirement multiplied by how long the pump runs³. Carry on with the made-up 60 watt pump. Ten minutes of running is a sixth of an hour, so about 10 Wh³. A pump is only on while water flows, so the minutes are what count.

Fitting it to a battery

Batteries are usually sold in amp-hours, written Ah. A steady current of one amp, kept up for an hour, moves one amp-hour of charge⁴. To compare it with watt-hours you need the battery's voltage too. Wikipedia says an average or nominal (labelled) voltage may be used for the estimate⁴. On that basis a 12 volt battery of 100 Ah holds about 1,200 Wh on paper.

Not all of that is yours to use. Three limits show up.

• Depth of discharge. Lead acid leisure batteries usually shouldn't be discharged below half of their rated capacity⁴. Many other batteries also have a limit on how low they can be run down⁴.
• Losses. Energy transfer is never 100% efficient, so some is lost on the way⁴.
• High currents. Pulling a lot of current from a battery can create heat and fire risk⁴.

Using the made-up numbers: half of 1,200 Wh leaves 600 Wh, and the 10 minute pump session uses about 10 Wh. That leaves room for dozens of sessions on paper⁴. Your real battery and pump will differ, so run the sums with your own figures.

If the pump will also run from panels, one guide suggests overestimating the power you need by roughly 1.25 to allow for losses¹. Driving a pump from a panel alone covers that case.

Wiring, fuses and when to call an electrician

Low voltage doesn't mean no care. Before any battery lead, fuse or cable goes on or comes off, switch the battery off from the circuit. Your pump's manual and your battery's manual come first. One pump maker puts isolating energy sources among its safety steps before diagnostic work⁵. If you're unsure, a licensed electrician should do it. Battery and fuse work isn't a routine DIY job.

Mains is a firm line. Anything fixed at 230V (wiring, switchboards, a grid connection) is licensed electrical work in Australia, never a beginner's project. New South Wales requires an electrical licence before any electrical wiring work, whatever the cost or type⁶. Western Australia makes it illegal for an unlicensed person to do electrical work, including changes to fixed wiring⁷. Rules come from each state, so check yours.

An inverter turns battery power into mains power. If your setup needs one, ask a licensed electrician to look at it.