How Much Does It Cost to Heat a Pool Per Month? 2026 Cost Guide

Heating a pool for a month costs whatever energy your heater uses to warm the water and replace ongoing heat losses, priced at your local utility rates. In a hypothetical U.S. residential example, a gas heater delivering 100,000 BTU/h at 80% thermal efficiency costs USD 9.38 for five hours of burner operation at USD 1.50 per therm. That works out to USD 112.50 for 12 identical heating sessions per month—not a national average or a quote for your pool.

Monthly Gas Heating Costs at Different Usage Levels

These are calculated examples for a hypothetical U.S. residential pool using 100,000 BTU/h of delivered heat, 80% thermal efficiency, and an assumed natural gas price of USD 1.50 per therm. The price is not a measured 2026 utility rate, and the efficiency is not a specification for a particular heater. Costs exclude circulation-pump electricity, heater auxiliary electricity, installation, maintenance, and additional fixed utility charges.

A five-hour swimming session does not necessarily mean five hours of burner operation. The thermostat may cycle the heater, and the circulation pump may run when the burner is off. For a useful monthly estimate, count heating energy or actual burner-on time rather than swimming hours.

Calculate Gas Heating Costs Without Double-Counting Efficiency

The first calculation decision is whether the heater’s BTU/h rating describes fuel input or heat output to the water. They are different quantities: thermal efficiency expresses how much of the fuel energy becomes useful heat. Manufacturer specifications should supply the rating and efficiency for your exact model; for example, Pentair lists different thermal efficiencies for its standard and heavy-duty MasterTemp heat exchangers.

The energy conversion is straightforward: one therm equals 100,000 BTU, according to the U.S. Energy Information Administration.

When You Know the Gas Input Rating

If the heater documentation identifies the rating as gas input:

[
\text{Gas use (therms)}
=
\frac{\text{Input rating (BTU/h)} \times \text{Burner-on hours}}{100{,}000}
]

[
\text{Fuel cost (USD)}
=
\text{Gas use (therms)} \times \text{Gas price (USD/therm)}
]

Do not divide by efficiency again. The input rating already describes the fuel energy consumed.

For a hypothetical 100,000 BTU/h input heater running for five hours at USD 1.50 per therm:

[
\text{Gas use}
=
\frac{100{,}000 \times 5}{100{,}000}
=
5\text{ therms}
]

[
\text{Session fuel cost}
=
5 \times 1.50
=
\text{USD }7.50
]

At 80% thermal efficiency, this heater would deliver 80,000 BTU/h to the water—not 100,000 BTU/h.

When You Know the Delivered Heat Output

If the rating describes useful heat delivered to the water, include thermal efficiency:

[
\text{Gas use (therms)}
=
\frac{\text{Heat output (BTU/h)} \times \text{Burner-on hours}}
{100{,}000 \times \text{Thermal efficiency}}
]

Use efficiency as a decimal: 80% becomes 0.80.

For the example in the opening table:

[
\text{Gas use per session}
=
\frac{100{,}000 \times 5}{100{,}000 \times 0.80}
=
6.25\text{ therms}
]

[
\text{Session fuel cost}
=
6.25 \times 1.50
=
\text{USD }9.375
\approx
\text{USD }9.38
]

For 12 identical sessions:

[
\text{Monthly fuel cost}
=
12 \times 9.375
=
\text{USD }112.50
]

Keep unrounded values until the final result. Multiplying the rounded session cost of USD 9.38 by 12 would slightly overstate the monthly total.

Common budgeting pitfall: “100,000 BTU heater” is incomplete information. If that number is fuel input, dividing by efficiency overstates consumption. The USD 9.38 example applies to 100,000 BTU/h of delivered heat at 80% efficiency.

Compare Gas and Heat Pumps for the Same Monthly Heat Demand

Comparing two heaters for the same number of operating hours can be misleading because they may deliver different amounts of heat per hour. Compare their costs for the same useful heat delivered to the pool instead.

A heat pump transfers heat from surrounding air into the water. Its coefficient of performance, or COP, is useful heat output divided by electrical energy input; a COP of 4 means that one kWh of electricity supplies four kWh of heat. Manufacturer guidance identifies outdoor temperature, pool conditions, and operating season as important influences on consumption.

Using EIA’s conversion of one kWh to 3,412 BTU, the simplified heat pump calculation is:

[
\text{Heat pump electricity (kWh)}
=
\frac{\text{Delivered heat (BTU)}}{3{,}412 \times \text{COP}}
]

[
\text{Heat pump cost (USD)}
=
\text{Electricity (kWh)} \times \text{Electricity price (USD/kWh)}
]

The 12-session gas example delivers:

[
100{,}000 \times 5 \times 12
=
6{,}000{,}000\text{ BTU per month}
]

At an assumed COP of 4 and an assumed electricity price of USD 0.20/kWh:

[
\text{Electricity}
=
\frac{6{,}000{,}000}{3{,}412 \times 4}
\approx
439.62\text{ kWh}
]

[
\text{Monthly heat pump cost}
=
439.62 \times 0.20
\approx
\text{USD }87.92
]

Monthly Energy Costs for 6 Million BTU of Delivered Heat

All rows are hypothetical U.S. residential budgeting scenarios, not national averages, local tariffs, or equipment guarantees. The calculation assumes a constant gas efficiency or heat pump COP over the month and excludes separate circulation-pump and auxiliary loads.

The comparison shows why a heat pump is not automatically cheaper under every combination of weather and utility prices. At the assumed rates, COP 3 costs slightly more than the 80%-efficient gas heater, while COP 4 costs less.

Match COP and Heating Capacity to Your Season

Do not use a favorable warm-weather COP for every month. Pentair’s UltraTemp installation guide identifies rated conditions of 80°F air, 80% relative humidity, and 80°F water, and explains that performance decreases as conditions fall below those conditions. Its performance data also distinguish heat output from electrical input.

Heating speed depends on available output, not simply the equipment category:

[
\text{Ideal heating time (hours)}
=
\frac{\text{Required heat (BTU)}}{\text{Delivered output (BTU/h)}}
]

A heat pump with lower delivered output than the gas heater being compared needs more operating time to supply the same heat. Real heating takes longer when heat escapes during operation, so check both seasonal COP and seasonal capacity before assuming the lower-cost option can reach your target temperature on schedule.

Why Monthly Costs Change Even With the Same Heater

Initial Warm-Up and Temperature Maintenance Are Different Loads

The first month of heating may include bringing cold water up to the desired swimming temperature. Subsequent heating replaces energy lost while maintaining that temperature. A monthly budget should account for both, rather than assuming that every heating session is identical.

A practical way to organize the calculation is:

[
\text{Monthly delivered heat}
=
\text{Initial warm-up heat}
+
\text{Heat needed to replace monthly losses}
]

If the pool cools between uses, each reheating period becomes part of the monthly demand. If you maintain the temperature continuously, the heater may operate between swimming sessions.

Covers and Wind Change the Heat You Must Replace

Evaporation removes heat from pool water, which makes covering an unused pool an important cost-control measure. DOE’s pool-heater analysis cites potential heating-energy savings of 50%–70% from pool covers; this is a reference range, not a guaranteed reduction for a particular pool.

Wind exposure, weather, water temperature, and how consistently the cover is used affect the losses that heating must replace. DOE’s pool-heater analysis accounts for pool conditions and operating patterns rather than treating heater size alone as a predictor of annual energy use.

A larger heater can consume more energy per operating hour while needing fewer hours to deliver a given amount of heat. At equal efficiency, delivering the same useful heat requires the same fuel energy in the simplified calculation. Heater size alone therefore cannot tell you the monthly bill.

Local Energy Prices Can Change the Comparison

Use the price applicable to your account and operating schedule. EIA publishes residential electricity prices by state and data month, but those averages are reference points—not the tariff for your property.

For electricity, include the usage-based supply and delivery charges that apply to the heater’s additional kWh, including time-of-use differences where applicable. For natural gas, use the applicable usage-based cost per therm, not just the commodity line on the bill.

Keep fixed customer charges separate. An existing monthly charge does not necessarily increase because you heat the pool; a new gas account established solely for heating may add a fixed cost that belongs in the project budget.

Build a Monthly Budget From Your Own Pool

1. Identify the equipment and rating basis.
   Record the model, fuel type, gas input or delivered output, thermal efficiency, or heat pump performance data. Confirm these against manufacturer documentation rather than an abbreviated sales listing.
2. Enter your local energy prices.
   Use USD/therm for the natural gas formulas and USD/kWh for electricity. Do not put a propane price per gallon or a gas price per cubic foot directly into a therm-based formula; the billing units must match the calculation.
3. Separate warm-up from ongoing heating.
   Include the initial temperature rise, reheating after cooling, and temperature maintenance between uses. This prevents a single session estimate from being mistaken for the full monthly requirement.
4. Estimate or record actual heating operation.
   For a fixed-rate gas heater, use burner-on hours. For modulating equipment, use measured energy or average input over the operating period rather than assuming full output throughout. For heat pumps, account for changing electrical input and COP.
5. Add heating-related electricity separately.
   Include extra circulation-pump runtime required for heating without counting hours already included in the normal filtration budget:

   [
   \text{Additional pump cost}
   =
   \text{Pump draw (kW)}
   \times
   \text{Additional hours}
   \times
   \text{Electricity price (USD/kWh)}
   ]

   Add separately measured heater auxiliary or standby consumption where relevant. DOE distinguishes active heating energy from standby and off-mode energy in its pool-heater analysis.

6. Reconcile the estimate with actual use.
   Compare the budget with heater logs, dedicated energy measurements, or utility consumption after accounting for other household loads. Update the assumptions for each heating month rather than treating one month as representative of the whole season.

Use the heater energy cost calculator to check the arithmetic, the pool heating time calculator to assess warm-up time, and the heat pump savings calculator to compare energy costs under matching assumptions.

Check the Estimate Before Using It

• The heater’s BTU/h rating is identified as input or delivered output.
• Efficiency is applied only when converting delivered heat into fuel consumption.
• Energy prices match the local account, billing units, and operating hours.
• Burner-on time is not confused with swimming time or circulation-pump runtime.
• Heat pump COP and output reflect the relevant seasonal conditions.
• The budget includes initial warm-up and heating between swimming sessions.
• Cover use and exposure conditions match the planned operation.
• Additional pump electricity and any new fixed charges are accounted for separately.
• Installation and maintenance are kept separate from monthly heating-energy costs.

The resulting figure is a planning estimate for the stated month and assumptions. It does not guarantee a water temperature or replace the heater manufacturer’s operating requirements.