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Pool Heater Buying Guide: Heat Pump vs Gas, and What Size You Need
Quick answer: Heat pumps cost far less to run and suit pools you heat continuously through the swim season; gas heaters cost more per hour but heat a pool fast regardless of ambient temperature, making them better for occasional or short-notice use. Heat pumps are sized by pool surface area, not volume — as a rough planning guide, a pool up to about 20 m² of surface needs roughly 8–11 kW output, 20–35 m² needs 13–16 kW, 35–50 m² needs 19–21 kW, and 50–70 m² needs 25–28 kW. Colder climates, exposed/windy sites, and pools without a cover all push you toward the larger end of the band.
Heat pump vs gas heater
| Heat pump | Gas heater | |
|---|---|---|
| Running cost | Low — draws ambient heat, uses electricity efficiently | High — burns gas directly per hour of use |
| Heat-up speed | Slow and steady, best left running continuously | Fast — can heat a pool in hours for a specific occasion |
| Cold weather performance | Efficiency drops as ambient air temperature falls | Unaffected by ambient temperature |
| Best suited to | Extending the swim season, running most days | Occasional use, short-notice heating, colder climates |
Some pools run both: a heat pump for day-to-day season extension, with gas as a fast top-up before a specific event. If you're only heating occasionally, gas running cost is less of an issue since it's not running every day.
Sizing a heat pump to your pool
Heat pump output is matched to your pool's surface area, because that's what determines how much heat the water loses to evaporation and the air — not the total volume. The table above is a planning guide only; brands vary in efficiency and rated output, so always check the specific unit's rated surface area coverage against your pool before buying.
What pushes you toward a bigger unit:
Climate. Cooler regions and larger day-to-night temperature swings mean more heat loss to make up, so pools in cooler climates are often sized up from the base recommendation.
Exposure and wind. An exposed, windy pool site loses heat faster through evaporation than a sheltered one, which increases the effective heating load beyond what surface area alone suggests.
Pool cover use. A pool cover significantly cuts evaporative heat loss when the pool isn't in use, which can meaningfully reduce the heater size needed or cut running time and cost on a correctly sized unit. If you're not planning to use a cover, size toward the top of the recommended band.
Desired temperature and season length. Wanting a warmer target temperature or heating earlier in spring and later into autumn both increase the heat pump's workload.
Sizing a gas heater
Gas heaters are typically chosen by desired heat-up time rather than a surface-area band — you're generally selecting the output (kW) that gets your pool to temperature within an acceptable window for how you use it, since a gas heater doesn't need to run continuously the way a heat pump does. Larger gas heaters heat a given pool faster but cost more to buy and run per hour; matching to genuinely occasional use rather than over-buying capacity is usually the more cost-effective choice.
FAQ
What size heat pump do I need for my pool? Base it on pool surface area, not volume — roughly 8–11 kW for pools up to 20 m² of surface, scaling up to 25–28 kW for 50–70 m², adjusted upward for cold climates, exposed sites, or pools without a cover.
Is a heat pump or gas heater cheaper to run? A heat pump is substantially cheaper per hour of operation since it moves ambient heat rather than generating it directly, but it needs to run for longer, steadier periods to be effective — gas costs more per hour but heats faster for occasional use.
Do I need a pool cover with a heat pump? Not strictly, but a cover meaningfully cuts evaporative heat loss and reduces running time and cost, especially overnight and on windy or cool days.
How long does a heat pump take to heat a pool? It varies with pool size, climate, and unit output, but heat pumps are designed for gradual, continuous heating rather than a fast single heat-up — expect it to run for extended periods to reach and hold your target temperature, particularly in cooler weather.