Pool Heater Sizing, BTU and Heat Pump vs Gas
A pool heater's BTU rating is the heat it adds per hour; bigger pools and colder air need more, gas warms faster, and a heat pump costs less to run.
By Karson AbernathyContributing writer.
On this page
The short answer
Size a pool heater by the heat it must deliver, which depends on how much water you have, how warm you want it, and how cold the air around the pool gets. A larger pool, a bigger jump in temperature or a cooler climate all call for more heating capacity. Then pick the type: Pentair explains that a gas heater warms water faster and holds up in cool weather, while a heat pump usually costs less to run over time but heats more slowly.
What does BTU mean on a pool heater?
BTU stands for British thermal unit. The U.S. Energy Information Administration defines one BTU as the heat needed to raise one pound of water by 1 degree Fahrenheit. A pool heater's rating is the number of BTUs it can put out in an hour, so a higher number means more heat in the same time.
That makes the physics simple, even if real pools are not. A heater that adds 400,000 BTU in an hour could in theory warm 400,000 pounds of water by one degree in that hour. In practice some heat escapes to the air, especially at night, in wind, and from an uncovered surface, so the water warms more slowly than the arithmetic suggests. Check whether a listing quotes input (fuel burned) or output (heat delivered to the water); the two differ by the heater's efficiency.
What size pool heater do I need?
Pentair names the factors to weigh: your climate, your pool size, what you will pay to run it, and how quickly you expect the water to warm. Its advice is that larger pools need more energy and may benefit from a heater with more capacity. A practical way to start:
- Note your pool's volume and whether it is above ground or in-ground.
- Decide how you swim: kept warm all season, or heated on demand for a weekend.
- Compare the heater's rated BTU with the pool volume the maker lists for that model, when it gives one.
On Wiki Shopping, 48 of the 114 pool heaters listed on October 26, 2026 have a title stating 250,000 BTU or more. Listings that state a size cluster at round figures: 10 say 400,000 BTU, 10 say 266,000 BTU and 7 say 399,000 BTU.
Heat pump vs gas pool heater
| Gas (natural gas or propane) | Heat pump (electric) | |
|---|---|---|
| How it heats | Burns fuel | Moves heat from the outside air into the water |
| Warm-up speed | Fast, suits on-demand heating and spas | Slow and steady |
| Cool weather | Keeps working | Works best when the air is warm |
| Purchase price | Usually lower | Usually higher |
| Running cost | Usually higher | Usually lower |
Raypak adds that a gas unit needs a gas line or a propane tank, a heat pump needs 220 volt electric service, and a heat pump's simpler, slower operation tends to mean less wear. Pentair also sells hybrid heaters that switch between the two methods, and rates its ETi 400 gas model at up to 96% thermal efficiency.
On Wiki Shopping, the listings that state a type break down as 42 natural gas, 23 propane, 28 heat pump and 9 solar.
How we count
Heater type and BTU rating are read from each pool heater's store listing title. Heaters that do not state them are left out of those counts. The counts change as listings are added or removed.
Shop by spec
Pool Heaters
Propane gas, 150000 Heating output, Propane
- Part Number: 014786
- Model Number: P-R156A-EP-C
- Black color
- Its very easy and elegant to use
Heat pump
Raypak Weatherking 6560 Pool Heat Pump TWPH-6560EHT01- Model Number: TWPH-6560EHT01
- Heating Capacity: 118,000 BTU
- Electrical Requirement: 208/230V, Single Phase, 60Hz
- Heat Exchanger: Spiral Titanium
- Compressor Type: High-Efficiency Scroll
Natural gas, 406000 Heating output, Natural gas
- Built to withstand various weather, great for environments requiring efficiency, convenience and versatility.
- Intermittent spark ignition pilot safety controls. Pilot only comes on when there is heat.
- Space-age material means the heater won't retain residual heat after it is turned off, allowing it to be installed without the use of metal heat sinks.
- Part Number: 009219 | Model Number: P-R406A-EN-C
- ProTek Shield module helps protect heat exchanger and extend life of heater
Natural gas, 150000 Heating output, Natural gas
- Part Number: 014784 | Model Number: P-R156A-EN-C
- Allows you to extend your swim season, open your pool earlier and close it later.
- Perfect choice for above-ground pools and spas
- 2" water connections
- Perfect for above-ground pools and spas