Pool heating cost starts with the energy needed to raise the water temperature, then depends on your heater’s efficiency and local energy rate. For an illustrative 20,000-gallon U.S. pool heated from 70°F to 80°F, the calculated natural gas cost is USD 30.51 at an assumed 82% thermal efficiency and USD 1.50 per therm, before accounting for heat lost during heating. A monthly or seasonal budget must also include maintaining the temperature and any additional circulation-pump electricity.
Illustrative Cost to Heat a 20,000-Gallon Pool by 10°F
| Heating method | Assumed efficiency or performance | Assumed energy price | Calculated purchased energy | Energy cost before pool heat loss |
|---|---|---|---|---|
| Natural gas heater | 82% thermal efficiency | USD 1.50/therm | 20.34 therms | USD 30.51 |
| Electric heat pump | Coefficient of performance, or COP, of 5.0 | USD 0.20/kWh | 97.77 kWh | USD 19.55 |
| Electric resistance heater | 100% conversion of electricity to heat for this simplified calculation | USD 0.20/kWh | 488.86 kWh | USD 97.77 |
These are hypothetical U.S. residential-pool examples, not 2026 national averages, state utility tariffs, equipment specifications, or installation quotes. All three rows use the same 1,668,000 BTU water-heating requirement and exclude pool heat loss, pump electricity, and other auxiliary loads. Energy conversions use 100,000 BTU per therm and approximately 3,412 BTU per kWh.
Calculate the Heat Your Pool Water Needs
The original calculation—gallons × 8.34 × temperature rise—estimates heat delivered to the water, not the amount of fuel purchased.
For ordinary pool-water temperatures, use the following approximation:
[
Q_{\text{water}}=V\times8.34\times\Delta T
]
Where:
- (Q_{\text{water}}) = heat required, in BTU.
- (V) = actual water volume, in U.S. gallons.
- (8.34) = approximate pounds of water per U.S. gallon.
- (\Delta T) = target temperature minus current temperature, in °F.
The formula uses an approximate water heat capacity of 1 BTU per pound per °F. Water density and heat capacity vary slightly with temperature, so this is a practical planning calculation rather than an exact laboratory result.
For the example pool:
[
\Delta T=80-70=10^\circ\text{F}
]
[
Q_{\text{water}}=20{,}000\times8.34\times10
=1{,}668{,}000\text{ BTU}
]
Use the 10°F temperature rise, not the final 80°F target. Also keep BTU and BTU/h separate: BTU measures an amount of heat, while BTU/h measures how quickly a heater supplies it.
Inputs to Collect Before Calculating
1. Determine the pool’s actual water volume at its operating water level.
- Measure the current water temperature and choose the target temperature.
- Identify the heater type and obtain its thermal efficiency or COP from the manufacturer’s documentation.
- Find your current utility or fuel price in the correct billing unit.
- Decide whether the estimate covers one warm-up, ongoing temperature maintenance, or an entire heating season.
The first two inputs determine the water-heating requirement. The remaining inputs determine how that heat requirement becomes purchased energy and a usable budget.
Convert BTU Into Natural Gas Cost
A therm contains 100,000 BTU, but a gas heater does not transfer all purchased fuel energy into the pool. Dividing by efficiency accounts for that difference. DOE evaluates active-mode gas pool-heater performance using thermal efficiency; use the value documented for your specific model rather than assuming all heaters perform alike.
[
\text{Gas use, therms}
=
\frac{Q_{\text{water}}}{100{,}000\times\eta}
]
[
\text{Gas cost, USD}
=
\frac{Q_{\text{water}}}{100{,}000\times\eta}
\times r_g
]
Where (\eta) is thermal efficiency expressed as a decimal and (r_g) is the applicable gas price in USD per therm.
Using the hypothetical inputs:
[
\text{Gas use}
=
\frac{1{,}668{,}000}{100{,}000\times0.82}
\approx20.3415\text{ therms}
]
[
\text{Gas cost}
=
20.3415\times1.50
\approx\text{USD }30.51
]
Simply dividing 1,668,000 BTU by 100,000 would produce 16.68 therms. That represents the water’s heat requirement expressed in therms, not the gas purchase required by an 82%-efficient heater.
Common budgeting pitfall: USD 30.51 is the calculated gas cost for this temperature increase with heater efficiency included, but pool heat loss excluded. It is not the price of keeping the pool at 80°F for a day, month, or season.
Use the Applicable Rate, Not the Whole Bill Average
For incremental heating cost, use the charges that increase when you consume another therm: applicable supply and delivery charges, plus usage-based taxes or adjustments. Keep a fixed monthly service charge separate if you would pay it even without heating the pool.
If your bill uses cubic feet or Ccf rather than therms, use the utility’s billed heat-content conversion. EIA explains that converting gas volume prices to energy prices requires the heat content of the gas; volume alone is not enough.
For propane, do not apply the natural gas therm calculation directly to a price quoted per gallon. Use the propane supplier’s applicable energy-per-gallon basis, delivered price, and the heater’s propane performance documentation.
Calculate Electric Heating Cost
Heat Pump Cost
A heat pump’s coefficient of performance is heat output divided by electrical energy input. A COP of 5.0 therefore means five units of heat delivered for each unit of electricity consumed; it does not mean that electricity is converted into heat at 500% efficiency. Heat pumps move heat from their surroundings into the water.
[
\text{Heat pump electricity, kWh}
=
\frac{Q_{\text{water}}}{3{,}412\times\text{COP}}
]
[
\text{Heat pump cost, USD}
=
\frac{Q_{\text{water}}}{3{,}412\times\text{COP}}
\times r_e
]
Here, (r_e) is the applicable electricity price in USD per kWh.
For the example:
[
\text{Electricity}
=
\frac{1{,}668{,}000}{3{,}412\times5.0}
\approx97.7726\text{ kWh}
]
[
\text{Cost}
=
97.7726\times0.20
\approx\text{USD }19.55
]
The COP of 5.0 is an assumption, not a verified specification for a particular heater. Use manufacturer performance data with its stated air and water test conditions, and do not assume one rated COP applies throughout the heating season.
Electric Resistance Cost
For the table’s simplified resistance-heating assumption:
[
\text{Electricity}
=
\frac{1{,}668{,}000}{3{,}412}
\approx488.8628\text{ kWh}
]
[
\text{Cost}
=
488.8628\times0.20
\approx\text{USD }97.77
]
These calculations compare energy cost for the same delivered heat. They do not compare installation cost, warm-up speed, or suitability for a particular site.
For time-of-use electricity plans, apply the rate for the hours when the heater actually operates rather than one blended price for every hour.
Separate Warm-Up Cost From Temperature Maintenance
The gallons-based formula answers a specific question: how much heat must be stored in the water to achieve a temperature increase?
Maintaining that temperature is a different problem. Pool heat losses include evaporation, convection, radiation, and conduction; outdoor solar gains can offset part of those losses. EPA’s ENERGY STAR technical reference also identifies wind, water temperature, outdoor air temperature, humidity, shading, and operating season as important sources of variation for outdoor-pool energy use.
That distinction explains why two pools with the same volume can have different monthly heating bills.
| Budget component | What to calculate or obtain | What it covers |
|---|---|---|
| Initial warm-up | Water volume × 8.34 × temperature rise, then adjust for heater performance and energy price | Heat stored in the water |
| Heat replacement | Site-specific heat-loss estimate or measured heater energy use | Losses during warm-up and temperature maintenance |
| Additional circulation | Pump input in kW × additional operating hours × electricity rate | Pumping attributable to heating |
| Maintenance | Equipment-specific service estimate or actual records | Service and repairs, separate from energy |
| Equipment and installation | Itemized local contractor quote | Upfront project cost, separate from operation |
EPA’s technical reference treats heating energy and circulation-pump electricity as separate components.
For a planning estimate, the total delivered heat over a period can be expressed as:
[
Q_{\text{period}}
=
Q_{\text{warm-ups}}
+
Q_{\text{net heat replacement}}
]
Convert that total into fuel or electricity using the appropriate heater formula. Net heat replacement should account for relevant heat gains as well as losses; do not automatically add an arbitrary percentage to the warm-up result.
Build a Monthly or Seasonal Budget From Actual Use
For an existing pool, measured energy use can provide a more useful operating budget than repeatedly calculating hypothetical temperature rises. EPA’s technical reference recommends submetering outdoor pools because their energy use varies substantially with local conditions.
- Record heater energy consumption for a defined period using a dedicated meter, reliable equipment monitoring, or another appropriately isolated measurement.
- Record the pool temperature setting, cover schedule, heater operating hours, and weather conditions.
- Multiply measured therms or kWh by the applicable rates for that period.
- Add additional pump electricity only if the measurement does not already include it.
- Compare periods with similar conditions before extending the results to a month or season.
If using runtime instead of metered energy, multiply operating hours by fuel input or electrical input—not delivered heating output. Account for changing input when equipment modulates.
A warm-weather week should not be treated as a reliable forecast for a cooler shoulder-season month. Likewise, multiplying the example’s one-time warm-up cost by 30 assumes a full 10°F recovery every day, which may not reflect your pool’s operation.
Account for the Cover Schedule
DOE’s Better Buildings pool-efficiency guidance identifies evaporation as a major energy-loss source and reports potential pool-cover heating-cost savings of 50%–70%. That is a general reference range, not a guaranteed discount for your pool or a multiplier to apply automatically to the warm-up table.
The practical budget input is how consistently the pool will actually be covered when not in use. Compare covered and uncovered operation through measured use or a site-specific estimate rather than assuming identical heat losses.
Check the Estimate Before Using It
- Water volume is in U.S. gallons, and temperature rise is in °F.
- The temperature rise is target temperature minus current temperature.
- Gas thermal efficiency is entered as a decimal, such as 0.82.
- Heat pump COP comes from documented performance conditions relevant to the estimate.
- Energy prices use matching units: USD/therm, USD/kWh, or the appropriate fuel unit.
- The rate’s utility territory and effective billing period are recorded.
- The estimate clearly separates one-time warm-up from ongoing maintenance.
- Pump electricity and heat-loss allowances are neither omitted nor counted twice.
- Equipment and installation costs come from an itemized local quote, not the energy calculation.
Use the result to plan and compare costs with transparent assumptions. Equipment operation, required water flow, installation, and service must still follow the specific manufacturer’s instructions and applicable local requirements.