Most people are surprised by which part of a pool costs the most to run — and by how much of it they can change without spending anything.
The pump is the whole bill, unless you heat
For an unheated pool, the pump is almost the entire running cost. It is the one thing that runs for hours every day. The arithmetic is plain: motor draw in kilowatts, times hours, times your rate.
A 1.5 HP single-speed motor draws somewhere around 1.5 to 2.0 kW in practice. Eight hours a day is therefore 12 to 16 kWh — put your own electricity rate against that and you have your daily figure.
Why variable speed changes the arithmetic
Pump power rises with roughly the cube of speed, while flow rises directly with it. Halve the speed and you get about half the flow for about an eighth of the power. Run it twice as long to move the same volume of water, and you have used roughly a quarter of the electricity for the same turnover.
That is the entire case for variable-speed pumps, and it is why many utilities offer rebates toward them and why several regions now require them on new installations.
Heating, if you heat
Heating swamps everything else. Raising water temperature costs about 8.34 BTU per gallon per degree Fahrenheit. A 20,000 gallon pool lifted 10 °F needs roughly 1.67 million BTU before losses — around 21 therms of gas once heater efficiency is taken into account.
Most of what you then pay for goes straight out of the surface as evaporation. This is why a cover, which is unglamorous and cheap, beats every thermostat adjustment you can make.
Calculators
The things worth doing first
- Work out your actual turnover requirement and stop running the pump past it. Most pools run longer than they need to.
- Use a cover if the pool is heated. Nothing else comes close.
- Keep the filter and baskets clean. A restricted system makes the pump work harder for less flow.
- Keep the water balanced. Stable chemistry costs far less than repeatedly correcting an unstable pool.
- Check your tariff. On time-of-use billing, moving the pump to off-peak hours costs nothing to do.
What is the single biggest saving available?
On an unheated pool, running the pump only as long as turnover requires. On a heated pool, a cover.
Is a variable-speed pump worth the money?
On most pools it pays for itself within a few seasons, because of the cube relationship between speed and power. The more hours your pump runs, the faster it pays back.
Does a bigger pump clean better?
No. It raises the power draw every hour it runs, and it usually pushes more flow than the filter and plumbing can use. Filtration gets worse, not better.
Why did my bill jump this summer?
Longer run times, more topping up, and faster chlorine consumption in warm water and strong sun. None of those are faults — but all three are worth measuring rather than guessing at.