Heating a pool, priced properly

Number one: heating the water once

This is the calculation people expect to be frightening and is not. It takes one British thermal unit to raise one pound of water by one degree Fahrenheit, and a US gallon of water weighs 8.34 pounds. There are 3,412 BTU in a kilowatt-hour.

Number two: the loss you are paying to replace

A pool is a large, shallow, uninsulated body of water sitting outdoors with an enormous surface area relative to its volume. It loses heat continuously, and the largest single mechanism is evaporation — turning water into vapor takes energy, and that energy comes out of the pool.

That loss runs all night, every night, and it is proportional to surface area, air temperature, humidity and wind. Which is exactly why we will not publish a degrees-lost-per-night figure: it depends entirely on your yard, and any national number would be fiction.

What is publishable is the consequence. If you are heating an uncovered pool, you are not paying the $18 above once — you are paying a fraction of it repeatedly, indefinitely, to replace what the surface gives away. That is where a heating bill comes from.

What a heater costs to run

Electric pool heaters publish their draw in kilowatts, and that is the number that matters rather than the purchase price. The arithmetic is the same one we use for pumps.

Sizing, which changes the bill more than the tariff does

A heater too small for the pool runs constantly and never reaches temperature, which is the worst of both worlds: maximum hours, no result. A heater sized properly reaches temperature and then cycles, which is far less time at full draw.

Sizing is a function of your gallons, the temperature rise you want and how fast you want it, and it interacts with the loss rate — which is why the cover comes first. Our heater sizing guide does that arithmetic, and the heater roundup covers what to look for on a spec sheet.

The cheaper interventions, in order

Because the loss term dominates, the measures that reduce loss beat the measures that add energy, and they cost a fraction as much.

A solar cover, used properly. Bubbles down, cut to size, on whenever the pool is not in use. Highest return of anything in this hub.

A reel. Not a luxury: it decides whether the cover is actually used past about week three, and an unused cover heats nothing.

Placement and windbreak. Free, decided once, and both meaningful — wind accelerates evaporation, which is the main loss.

Running the pump in daylight. Small, but free if you were running it anyway. Our run-time guide covers scheduling it.

Our season extension guide is the practical version of this list, and the energy saving guide ranks every measure on this site by what it actually returns.

Is it worth it?

The honest framing is the one from our cost per swim guide: a heater raises the numerator substantially and also raises the denominator, because a warm pool in May and September is a pool that gets used on days it otherwise would not.

For a household that would genuinely swim through the shoulder months, that trade can work. For a household that uses the pool on hot afternoons anyway, a heater is money spent to make a warm pool warmer, and the cover does that for a fraction of the price. There is also a point in autumn where you are heating against genuinely cold nights and the bill runs away — recognizing that point is worth more than any tariff.