A pool costs money in four places: the electricity to move the water, the fuel to heat it, the water itself, and the chemicals that keep it sanitary. Only one of those is usually large, and which one depends on whether the pool is heated.
Where the money actually goes
For an unheated pool, the pump is almost the whole bill. It is the one piece of equipment that runs for hours every day, and it is the only one whose cost you can change substantially without spending anything.
For a heated pool, heating dwarfs everything else — often by more than the other three combined. The arithmetic below shows why.
| Cost | What drives it | How much you can change it |
|---|---|---|
| Pump electricity | Motor power × hours run × your rate | A great deal — see below |
| Heating | Temperature rise × volume × fuel price | A great deal, mostly with a cover |
| Water | Volume, plus evaporation and backwashing | A little |
| Chemicals | Volume, sunlight, bather load | A little, through stable water balance |
Pump electricity, and why speed matters so much
The cost is simply the motor draw in kilowatts, multiplied by the hours it runs, multiplied by your electricity rate. A 1.5 HP single-speed motor draws roughly 1.5 to 2.0 kW in practice, so eight hours a day is somewhere between 12 and 16 kWh.
What makes variable-speed pumps worth their price is that pump power follows roughly the cube of speed, while flow follows speed directly. 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 water and you have used roughly a quarter of the electricity.
This is also why a bigger pump is not a better pump. Oversizing raises the power draw every single hour the pump runs, and the extra flow is often more than the filter and plumbing can use.
Heating, and the one thing that changes it
Raising water temperature takes a fixed amount of energy: about 8.34 BTU for every gallon, for every degree Fahrenheit. A 20,000 gallon pool lifted by 10 °F therefore needs roughly 1.67 million BTU — before any losses. At 100,000 BTU per therm, and allowing for a gas heater running at around 80% efficiency, that is on the order of 21 therms of gas.
The losses are the real story. Most of the heat a pool loses goes out through the surface, and most of that is evaporation rather than radiation. A cover addresses exactly that loss, which is why it is the single most effective thing you can do about heating costs — far more than adjusting the thermostat by a degree.
A heat pump changes the arithmetic again: it moves heat rather than making it, so it delivers several units of heat per unit of electricity. It is slower than a gas heater and it stops being efficient once the air turns cold, which is why the two suit different climates.
Water
Filling is a one-off: your volume in gallons, divided by a thousand, multiplied by whatever your utility charges per thousand gallons. Topping up is the ongoing part, and it comes from evaporation, splash-out and backwashing. A cover reduces the first of those as well.
Chemicals
Chemical spend tracks volume, sunlight and how much the pool is used. The largest avoidable cost here is chlorine burned off by UV, which is what cyanuric acid exists to prevent — and it is also why letting stabiliser climb too high costs money in the other direction, because a high CYA reading raises the chlorine level you have to hold.
Work out your own numbers
What these estimates do not include
Every calculator here works from the figures you enter — your pool, your rates, your run times. They do not attempt to price repairs, replacement parts, opening and closing, or professional servicing, because those vary far more by region and by pool than the running costs do. Treat the results as running costs, not as a total cost of ownership.
Is it cheaper to run the pump at night?
Only if your electricity is billed at time-of-use rates, in which case it can be a substantial saving. On a flat rate the time of day makes no difference to cost — though running during daylight does let chlorine work while the sun is actively burning it off.
Does running the pump fewer hours save money?
Yes, proportionally — but only down to the point where the water still turns over completely. Below that the water stops being filtered properly and you spend the saving on chemicals instead. The turnover calculator shows where that floor is for your pool.
How much does a pool cover actually save?
It depends on climate and how often the pool is used, so we do not publish a single figure. What is not in doubt is the mechanism: most heat loss is evaporation from the surface, and a cover stops it. On a heated pool it is the highest-return purchase available.
Why is my bill higher in summer if I am not heating?
Longer pump run times and more frequent topping up. Warmer water also consumes chlorine faster, and stronger sunlight destroys more of it.