Pool Heating Guide: Gas, Heat Pump and Solar Compared (2026)

Gas pool heaters suit fast, occasional heating; heat pumps suit regular temperature maintenance when outdoor conditions support efficient operation; and solar suits pools with good sun exposure and flexible heating schedules. The right choice depends on how quickly you need warm water, how long your swimming season lasts, and what energy costs locally—not efficiency alone. Compare each system’s usable heat output and site requirements, then account for the heat your pool loses between swims.

Gas, Heat Pump and Solar: Which Fits Your Pool?

These are selection patterns, not universal rankings. A short swimming season does not automatically make gas the cheapest option, and a heat pump is not automatically cheaper than natural gas. The useful comparison is the cost of delivering the same amount of heat under conditions you actually expect.

Start With the Temperature Rise and Heating Deadline

Before comparing equipment, distinguish initial warm-up from temperature maintenance. Raising a cold pool by 10°F requires a defined amount of stored heat; keeping it warm requires replacing continuing losses. Pool volume drives the warm-up calculation, while surface exposure and weather strongly affect ongoing demand. Evaporation is the largest source of pool energy loss, according to the U.S. Department of Energy.

Use this sequence to establish a comparable heating requirement:

1. Determine the actual water volume in U.S. gallons.
2. Measure the starting water temperature and choose the desired temperature.
3. Calculate the temperature rise: target temperature minus starting temperature.
4. Set the heating deadline in hours.
5. Calculate the ideal heat requirement, then have the equipment selection checked against expected heat losses and manufacturer performance data.

The simplified water-heating relationship is:

[
Q = V \times 8.34 \times \Delta T
]

Where:

• (Q) = heat added to the water, in BTU.
• (V) = pool volume, in U.S. gallons.
• (8.34) = approximate pounds of water per U.S. gallon.
• (\Delta T) = temperature rise, in °F.

The formula uses an approximate water heat capacity of 1 BTU per pound per °F. It estimates energy stored in the water, not fuel consumption or heat lost during the process; the BTU definition provides the basis for this calculation.

Worked Example: A 20,000-Gallon Pool

Assume a hypothetical pool starts at 70°F and needs to reach 80°F within 12 hours. Ignore heat loss initially.

[
\Delta T = 80-70 = 10^\circ\mathrm{F}
]

[
Q = 20{,}000 \times 8.34 \times 10
= 1{,}668{,}000\ \mathrm{BTU}
]

Required average heat output:

[
P_{\mathrm{out}} = \frac{1{,}668{,}000}{12}
= 139{,}000\ \mathrm{BTU/hr}
]

That is the ideal heat delivery requirement at the water—not necessarily the number printed in a heater’s model name.

For a gas heater with an assumed thermal efficiency of 84%:

[
P_{\mathrm{in}} = \frac{139{,}000}{0.84}
\approx 165{,}476\ \mathrm{BTU/hr}
]

This is an ideal minimum gas input before allowing for pool heat loss. It is not a finished equipment recommendation.

Common sizing mistake: Output and input are different quantities. Divide the required water-side output by thermal efficiency when estimating gas input. Do not apply that same adjustment to a heat pump’s published water-heating output; its COP describes the electricity needed to produce that output.

If average heat loss can be estimated, a more useful planning relationship is:

[
P_{\mathrm{out,required}}
= \frac{Q}{t} + P_{\mathrm{loss}}
]

This still simplifies changing weather and operating conditions. A deadline-sensitive project needs a site-specific assessment rather than an arbitrary percentage added to the ideal calculation.

For a hand-checkable starting estimate, use the Heater BTU size calculator.

Gas: Prioritize Recovery Speed and Fuel Supply

Gas heaters burn natural gas or propane to produce heat. They are a practical choice when you want to heat shortly before use rather than maintain temperature throughout the week; DOE guidance identifies quick, short-period heating as a suitable application.

Efficiency must come from the exact model and configuration. Pentair’s 2025–2026 catalog lists standard MasterTemp models at 84% thermal efficiency and certain heavy-duty versions at 82%. It lists the ETi 400 at 96%, illustrating why one efficiency assumption should not represent every gas heater.

For the hypothetical pool above, assume a gas heater has:

• Fuel input: 400,000 BTU/hr.
• Thermal efficiency: 84%.

Estimated water-side output:

[
400{,}000 \times 0.84 = 336{,}000\ \mathrm{BTU/hr}
]

Ideal heating time:

[
t = \frac{1{,}668{,}000}{336{,}000}
\approx 4.96\ \mathrm{hours}
]

The speed comes from available output, not the word “gas” alone. Compare actual output when evaluating differently sized systems.

Installation can change the financial decision. Confirm the correct fuel model, supply capacity, gas piping, venting where applicable, and manufacturer installation requirements before treating an equipment price as the installed project cost. Pentair specifically directs installers to its engineering data for heater selection and gas supply sizing.

Heat Pump: Compare Cool-Weather Output, Not Just Advertised COP

A heat pump transfers heat from outdoor air into pool water. Its coefficient of performance, or COP, is:

[
\mathrm{COP}
= \frac{\text{heat delivered}}{\text{electrical energy consumed}}
]

Both quantities must use the same energy units. A COP of 5 means five units of heat delivered for each unit of electricity consumed; it is not a guarantee that heating will cost one-fifth as much as gas, because electricity and gas have different prices.

Why Rating Conditions Matter

Pentair’s 2025–2026 catalog provides the following performance values for the UltraTemp 140. The conditions are listed as air temperature / relative humidity / water temperature.

These are model-specific published ratings, not national averages or predictions for every pool. The same unit has less capacity and lower efficiency under the cooler test condition.

For frequent swimmers, a heat pump can therefore be a strong candidate for steady heating, but its shoulder-season performance deserves particular attention. Check the model’s output at cooler conditions, minimum operating temperature, defrost behavior, and installation requirements rather than assuming a universal temperature cutoff.

In a separate hypothetical comparison, a heat pump delivering 140,000 BTU/hr would take:

[
t = \frac{1{,}668{,}000}{140{,}000}
\approx 11.91\ \mathrm{hours}
]

That is slower than the example gas heater because its output is lower. It does not establish that every heat pump is slower than every gas heater.

Solar: Size Collectors for the Site and Swimming Season

Solar thermal pool heating circulates water through collectors that capture sunlight. It can have very low annual operating costs, but available sunshine and system design determine how much heat it supplies. Solar thermal collectors are different from photovoltaic panels that generate electricity.

Collector area should be related to pool surface area, not determined from gallons alone. The Florida Solar Energy Center primer gives illustrative collector-to-pool-area ratios of roughly 65% in southern Florida, 80% in central Florida, and 100% in northern Florida, while emphasizing that requirements can vary significantly. These are regional reference points, not a universal sizing rule.

For a hypothetical rectangular pool measuring 16 ft by 32 ft:

[
A_{\mathrm{pool}} = 16 \times 32 = 512\ \mathrm{ft^2}
]

If a site-specific design calls for an 80% collector ratio:

[
A_{\mathrm{collector}} = 512 \times 0.80
= 409.6\ \mathrm{ft^2}
]

The assumed 80% ratio is an example, not a prescription. Collector performance, shading, climate, desired season length, and the target water temperature need to support the final design.

Do not treat solar as a fixed-output heater that operates at its peak rating all day. Ask for expected seasonal heat delivery and backup requirements if you need a dependable temperature by a particular time. Also include any additional circulation electricity and maintenance in the budget; free sunlight does not mean every system expense is zero.

Use the Solar heating size calculator to check the area arithmetic before requesting a site-specific design.

Compare Heating Time After Allowing for Heat Loss

The no-loss heating-time formula is:

[
t = \frac{Q}{P_{\mathrm{out}}}
]

With an assumed constant average heat loss:

[
t = \frac{Q}{P_{\mathrm{out}}-P_{\mathrm{loss}}}
]

For the same 20,000-gallon, 10°F-rise example, assume an illustrative average loss of 20,000 BTU/hr. This loss is a modeling assumption, not a typical value or site measurement.

For the heat pump:

[
t = \frac{1{,}668{,}000}{140{,}000-20{,}000}
= 13.90\ \mathrm{hours}
]

If heat loss equals or exceeds heat output, the pool cannot achieve a sustained temperature rise under those conditions. Real heat loss and heat-pump output change over time, so these constant-rate calculations are planning estimates rather than heating guarantees.

Check alternative outputs and temperature rises with the Heating time calculator.

Compare Energy Costs Using Your Local Rates

Compare the cost per unit of useful heat before comparing seasonal bills. EIA lists 1 therm as 100,000 BTU and 1 kWh as approximately 3,412 BTU.

For natural gas:

[
C_{\mathrm{gas}}
= \frac{Q}{100{,}000 \times \eta}
\times R_{\mathrm{gas}}
]

For a heat pump:

[
C_{\mathrm{HP}}
= \frac{Q}{3{,}412 \times \mathrm{COP}}
\times R_{\mathrm{electricity}}
]

Here, (\eta) is gas thermal efficiency as a decimal, (R_{\mathrm{gas}}) is USD per therm, and (R_{\mathrm{electricity}}) is USD per kWh. Propane requires a separate calculation using its delivered price and fuel-energy basis; do not insert a USD-per-gallon price into the natural-gas formula.

Hypothetical U.S. Cost Example

Assume a hypothetical U.S. residential pool, with no specific state represented, uses these illustrative inputs:

• Natural gas: USD 2.00 per therm.
• Electricity: USD 0.20 per kWh.
• Gas thermal efficiency: 84%.
• Heat-pump COP: 5.
• Water-heating requirement: 1,668,000 BTU, excluding pool heat loss.

These are not 2026 national averages, utility tariffs, or quotations.

Gas consumption and cost:

[
\frac{1{,}668{,}000}{100{,}000 \times 0.84}
= 19.86\ \mathrm{therms}
]

[
19.86 \times 2.00 \approx \mathrm{USD}\ 39.71
]

Heat-pump electricity and cost:

[
\frac{1{,}668{,}000}{3{,}412 \times 5}
= 97.77\ \mathrm{kWh}
]

[
97.77 \times 0.20 \approx \mathrm{USD}\ 19.55
]

Under these assumptions, the heat pump’s heating-energy cost is about half the gas cost. That comparison changes with local prices and operating COP, and it excludes circulation electricity, maintenance, installation, and heat losses.

Use current marginal energy rates, including applicable variable delivery charges. Account separately for any new fixed charges caused by adding a service. For a seasonal budget, include initial warm-up, reheating after shutdowns, and continuing temperature maintenance—not just one session multiplied by swim days.

Use the Heater cost calculator and Heat pump savings calculator to compare the same heating requirement under your own rates.

A Cover Can Change the Equipment Decision

DOE identifies evaporation as the largest source of pool energy loss and reports potential heating-cost savings of 50%–70% from pool covers. That range is a reference opportunity, not a guaranteed reduction for an individual pool.

Cover use matters because heat retained does not need to be purchased again. A regularly covered pool may place less recovery demand on a heat pump or solar system than an otherwise comparable uncovered pool. Compare heater options using the cover schedule you will actually follow, especially between swims and overnight, rather than assuming ideal cover use in the estimate.

Check the Complete Installation Before Choosing

Compare installed scope and expected operation, not equipment price alone. Manufacturer performance tables and installation documents should govern model-specific decisions; local approval and utility requirements must also be checked for the actual project.

• Pool volume, starting temperature, target temperature, and heating deadline are recorded.
• Gas input and water-side heat output are clearly distinguished.
• Heat-pump output and COP are checked at relevant air, humidity, and water conditions.
• Solar collector space and expected seasonal performance are assessed for the site.
• The estimate uses the planned cover schedule and accounts for heat loss.
• Energy prices come from the current local tariff or fuel supplier.
• Installed quotes identify equipment, labor, utility work, plumbing, controls, and applicable approval costs.
• Fuel supply, electrical capacity, water flow, clearances, and venting are checked against the exact model’s instructions.

For occasional, deadline-driven heating, prioritize sufficient recovery output. For regular swimming, compare the cost of maintaining temperature throughout the season. For solar, establish whether available collector space and sunshine can meet the intended schedule before deciding how much backup heating is needed.