Pool Heater vs Heat Pump: Which Is Cheaper to Run?
Gas heaters and heat pumps both warm pool water, but they do it in completely different ways, and that changes what they cost to run, how fast they work and which climates they suit. The short answer: a heat pump is usually cheaper per degree when the air is mild, and a gas heater wins when you need heat fast or in cold weather. Here is how to tell which applies to you.
How each one works
A gas or propane heater burns fuel and passes the pool water through a heat exchanger. Efficiency is limited by physics: US federal rules require at least 82% thermal efficiency, and even the best units deliver less heat than the energy in the fuel they burn.
A heat pump works like an air conditioner in reverse. It uses electricity to move heat from the outside air into the water. Because it moves heat rather than making it, it delivers several units of heat for each unit of electricity, a ratio called the coefficient of performance (COP). The U.S. Department of Energy uses a COP of 5 in its example: five units of heat for one unit of electricity.
Cost per unit of heat
The fairest comparison is what it costs to put the same amount of heat into the water:
| Heater | Energy price used | Per 100,000 BTU into the water | 20,000 gal, +10 °F |
|---|---|---|---|
| Natural gas heater (82% efficient) | $1.56/therm | $1.90 | $31.73 |
| Propane heater (82% efficient) | $2.73/gal | $3.64 | $60.72 |
| Heat pump (COP 5) | $0.18/kWh | $1.07 | $17.79 |
Prices: electricity U.S. Energy Information Administration, Short-Term Energy Outlook (2026-10-29); natural gas U.S. EIA residential natural gas price series N3010US3; propane U.S. EIA Heating Oil and Propane Update. Heat losses during heating are not included. Replace with your own rates in the calculators.
With these prices, the heat pump delivers heat for far less than propane and noticeably less than natural gas. Your result depends on your local prices: cheap natural gas and expensive electricity narrow the gap, while propane is usually the most expensive way to heat a pool. The US average electricity price in our data is $0.182 per kWh; replace it with your own rate in the heat pump calculator.
Speed: gas is much faster
Heat pumps are smaller in output. Standard residential sizes are 75,000 to 125,000 BTU per hour of heat, while gas heaters commonly come in 200,000 to 400,000 BTU per hour.
Warming 20,000 gallons by 10 °F takes 20,000 × 8.34 × 10 = 1,668,000 BTU. A 400,000 BTU/h gas heater at 82% efficiency delivers about 328,000 BTU/h, so it needs around 5 hours. A 100,000 BTU/h heat pump needs about 17 hours of running, before any heat losses. In practice heat pumps are left running to hold a temperature, while gas heaters can heat the pool for a weekend on demand.
Climate matters for heat pumps
A heat pump pulls heat from the air, so its output and efficiency fall as the air gets colder. They work best when the air is above about 50–60 °F. In a cool spring or autumn the COP drops and the heat pump may not keep up. Gas heaters are unaffected by air temperature.
Which one should you choose?
| Your situation | Usually better |
|---|---|
| Pool kept warm for the whole season | Heat pump |
| Warm or mild climate | Heat pump |
| Occasional heating for weekends or parties | Gas heater |
| Swimming in cold weather or extending into winter | Gas heater |
| Only propane available, high use | Heat pump (propane is costly) |
| Spa that must heat quickly | Gas heater |
Some owners install both: a heat pump for steady season-long heating and a gas heater for fast boosts.
Size it correctly
An undersized heater of either type runs longer and may never reach the target on cool days. Both calculators use the U.S. Department of Energy’s sizing method, based on surface area and the gap between the water temperature you want and the average air temperature of your coolest swim month:
- Pool heater size calculator for gas and propane.
- Pool heat pump size calculator for electric heat pumps, including running cost per hour.
The cheapest heat is heat you keep
Most of the heat a pool loses leaves through evaporation at the surface. A cover blocks most of that, and DOE puts heating savings from a pool cover at 50–70%. Before choosing between gas and a heat pump, estimate what a cover would save with the solar cover savings calculator, and see the full yearly picture with the operating cost calculator.