Heating a pool

There are five ways to put heat into a swimming pool, and the honest version of the comparison is not the one usually published. Most articles rank heaters by purchase price and running cost separately, which lets the two cancel each other out in the reader’s head and resolves nothing. The number that decides the argument is what it costs to deliver a million BTU into the water, because that is the unit of the actual job.

At US average residential energy prices, and at the equipment efficiencies we publish and let you change, that number comes out like this: solar costs nothing to run. An electric heat pump costs about $10.75. Natural gas costs about $28.36. Propane costs about $35.66. An electric resistance heater costs about $53.75 — five times the heat pump, on the same electricity.

That spread surprises people because fuel price alone points the other way. Electricity is dear per unit of energy bought and natural gas is cheap. The reversal happens because a heat pump does not make heat, it moves it, delivering roughly five units of heat for every unit of electricity it draws. Once you divide through by efficiency, the ranking inverts. Dividing through by efficiency is the whole job, and it is what the cost pages are for.

So which one should you buy

The running cost is not the only axis, and pretending it is would be its own kind of dishonesty. Three trades decide it:

Speed against cost. A gas heater will lift a 20,000-gallon pool ten degrees in about five hours. A heat pump of a typical size takes most of a day. That comparison comes down to whether you hold a temperature all season or heat a weekend pool on Friday afternoon. If it is the latter, gas is the right answer even though it costs more per BTU.

Control against free. Solar panels cost nothing to run, forever, and that is not a small thing. What you give up is the thermostat: you get whatever the sun delivered that week, and on a cold cloudy stretch — exactly when you want heat most — they deliver very little.

Adding heat against keeping it. This is the one the category skips. A hundred-dollar cover and a three-thousand-dollar heater are not competing products, but for a lot of pools the cover does more of the job. If your pool is dropping several degrees overnight, you are buying a heater to replace heat you could simply have kept. Doing it with no heater at all is a real strategy, not a consolation prize.

The sizing trap

One thing worth knowing before you shop, because it costs people real money: gas heaters are rated by fuel INPUT and heat pumps by heat OUTPUT. A 400,000 BTU gas heater burns 400,000 BTU an hour and delivers roughly 82% of it. A 140,000 BTU heat pump delivers 140,000 BTU. Compare those two numbers directly — which is what the listings invite you to do — and you will badly over-rate the gas heater. The sizing guide works through what you actually need.

Below: the full walkthrough of how to heat a pool, the ranked solar heater picks, the head-to-head comparisons, and the cheap approaches that work better than their price suggests. If you want a number for your own pool rather than a general answer, the running-cost calculator takes your volume, your rate and your equipment and shows its working.