Pool heater vs heat pump (and solar, and a cover)

The four options in brief

The energy every option has to supply

Whatever heats the pool, the water needs the same heat. One BTU warms one pound of water by 1°F, and a US gallon of water weighs about 8.34 lb, so warming 15,000 gallons by 10°F takes 15,000 × 8.34 × 10 = 1,251,000 BTU. That is before losses, which run the whole time the heater works.

heat (BTU) = gallons × 8.34 × °F rise
gas (therms) = heat ÷ efficiency ÷ 100,000
propane (gal) = heat ÷ efficiency ÷ 91,452
heat pump (kWh) = heat ÷ COP ÷ 3,412
electric resistance (kWh) = heat ÷ 3,412

The fuel figures come from the US Energy Information Administration: a therm of natural gas is 100,000 BTU, a gallon of propane about 91,452 BTU, and a kWh 3,412 BTU. Everything else is division.

A worked cost comparison

Here is that one 10-degree heat-up of 15,000 gallons priced six ways. The prices are examples, not averages — $1.50 per therm, $2.50 per gallon of propane and $0.17 per kWh — so put in the rates from your own bills.

At these example prices a heat pump at COP 5 delivers heat for about $9.96 per million BTU, against $18.29 for an 82% gas heater and $33.34 for propane. But look at the time column: the gas heater does the job in about 6.1 hours, the heat pump in about 12.5 hours on paper, and longer in practice because its output falls as the air cools. On a cool night its COP may be nearer 3, which raises its cost to about $20.78 for the same heat-up — still competitive, but the gap narrows.

When does a heat pump beat gas?

Per BTU delivered, a heat pump is cheaper whenever the price of a kWh divided by (3,412 × COP) is less than the price of a therm divided by (100,000 × efficiency). Rearranged:

heat pump is cheaper when: price per kWh < price per therm × 3,412 × COP ÷ (100,000 × gas efficiency)

With gas at an example $1.50 per therm and an 82% heater, the heat pump wins at COP 5 whenever electricity costs less than about $0.31 per kWh, and at COP 3 below about $0.19. Swap in your own prices. Propane is cheaper than natural gas per BTU only when a gallon costs less than about 0.91 times a therm (91,452 ÷ 100,000).

Sizing: the DOE formula

The DOE sizes both gas heaters and heat pumps the same way: surface area in square feet × the temperature rise × 12, where the rise is the pool temperature you want minus the average air temperature of the coldest month you plan to swim. For a 16 × 32 ft pool (512 sq ft) kept at 82°F through a month averaging 65°F, that is 512 × 17 × 12 = 104,448 BTU/h of heat output. The DOE says the formula assumes a 1 to 1-1/4°F rise per hour and a 3-1/2 mph average wind. If you also want a quick first heat-up, the heater has to cover both at once: warming 20,000 gallons by 17°F in 24 hours adds about 118,000 BTU/h, which is why the heater calculator suggests a 300,000 BTU/h gas heater for that pool rather than the formula figure alone.

Two things trip people up. Gas heaters are usually rated by gas input, so divide by efficiency to compare (a 250,000 BTU/h heater at 82% puts about 205,000 BTU/h into the water). Heat pumps are rated by output, but at 80°F air; standard sizes are 75,000, 100,000 and 125,000 BTU/h, and gas heaters run from 75,000 to 450,000. Worked through on what size pool heater for 20,000 gallons and how long to heat a pool 10 degrees.

Heat-up time, and why it matters

If you heat the pool for a weekend and let it cool between, speed is the whole point, and gas wins: it can bring a pool up in an afternoon or overnight, in any weather. If you keep the pool warm all season, a heat pump’s slow, cheap output holds temperature well, and the first heat-up is a one-off wait. The DOE notes heat pumps work efficiently only above about 45–50°F, so in spring and fall in cool climates they slow down just when you need them.

Solar is slower still and depends on sun, but its fuel is free. The DOE suggests collector area of 50–100% of the pool’s surface — for a 512 sq ft pool, roughly 256 to 512 sq ft of collectors — more in cooler or cloudier places, or for a longer season. Many owners pair solar with a small heater for cloudy spells.

Before buying any heater: the cover

The DOE calls evaporation by far the largest source of a pool’s heat loss: each pound of 80°F water that evaporates carries off 1,048 BTU. That is why it puts a cover’s heating savings at 50–70%, and why a cover shortens every heat-up and lets a smaller heater hold temperature. A cover burns no fuel at all. How many gallons per inch in a 24 ft round pool shows how much water — and heat — an inch of evaporation is, and the solar cover calculator sizes one.

• Set it lower. The DOE says the usual range is 78–82°F and each extra degree costs 10–30% more energy.
• Heat for the days you swim, especially with gas; holding temperature all week for a Saturday swim pays for five days of losses.
• Block the wind. Wind speeds evaporation; a fence or hedge on the windward side helps.

What it adds to the monthly bill

The heat-up cost above is a floor. The real bill depends on how many hours a day the heater runs to make up losses, which depends on weather, wind and the cover. What a heat pump pool heater costs per month works one through at 8 hours a day; how much it costs to run a pool per month puts heating next to the pump, chemicals and water. More in pool running costs.