Pool Heating Cost Calculator
Estimate what it costs to heat a swimming pool across a season on gas, propane or a heat pump, and see how much of that a cover saves by suppressing evaporation.
How to use this calculator
- 1Get the surface area right — it is the number the cost actually depends on. Volume only affects the one-off heat-up.
- 2Use the 24-hour average air temperature for the season, not the daytime high. Nights are when the pool loses most of its heat.
- 3Set the loss coefficient to reflect your site: lower if the pool is sheltered and the air humid, considerably higher if it is exposed and windy.
- 4Model the cover honestly. Overnight alone is 12 to 14 hours and captures most of the available saving without costing any swimming time.
- 5Compare the last table before comparing heaters. A cover and a lower thermostat setting both beat any change of equipment, and neither has a running cost.
How the calculation works
Heat-up (BTU) = gallons x 8.34 x (target − starting temperature)
Standing loss (BTU/hr) = surface area x (target − air temperature) x loss coefficient
Daily loss = uncovered hours x standing loss + covered hours x standing loss x (1 − cover reduction)
Season energy = heat-up + daily loss x days
Cost = season energy ÷ efficiency ÷ BTU per fuel unit x unit price- 8.34
- Pounds in a US gallon of water. With water’s specific heat of 1 BTU per pound per °F, this is the whole of the heat-up calculation
- Loss coefficient
- BTU lost per hour per square foot per °F of pool-to-air difference. About 5 for an average site; lower when sheltered and humid, much higher when windy and dry
- Cover reduction
- The share of surface loss a cover eliminates, chiefly by stopping evaporation. DOE puts the resulting heating saving at 50–70%
- Efficiency
- Thermal efficiency for a combustion heater, or COP expressed as a percentage for a heat pump — a COP of 5 is 500%
Surface area, not volume, dominates the cost. A deep pool and a shallow one of the same surface area cost nearly the same to maintain, because the heat leaves through the surface — the deeper one only costs more once, at heat-up.
The loss coefficient bundles evaporation, convection and radiation into one figure, and evaporation is roughly 70% of it. Because evaporation depends on wind speed and humidity rather than temperature alone, the coefficient genuinely varies from site to site, which is why it is an input rather than a constant.
Heat-up and maintenance are reported separately because they answer different questions. Heat-up decides what size heater to buy; maintenance decides what the season costs. Confusing the two is why people buy a large heater and are then surprised by the bill.
Worked example
A 512 sq ft, 21,065 gallon pool held at 82°F on gas for a 120-day season
- 1.Heat-up: 21,065 gallons x 8.34 lb x 20°F rise = 3.51 MMBtu.
- 2.Standing loss: 512 sq ft x (82 − 70) = 12°F x 5 BTU/hr/sqft/°F = 30,720 BTU an hour.
- 3.Over 24 hours for 120 days uncovered: 30,720 x 24 x 120 = 88.5 MMBtu.
- 4.Total: 3.51 + 88.5 = 92.0 MMBtu of heat into the water — maintenance is 96% of it.
- 5.At 84% efficiency the heater burns 92.0 ÷ 0.84 = 109.5 MMBtu of gas, which is 1,095 therms.
- 6.At $1.50 a therm that is $1,643 for the season.
- 7.Covering overnight for 14 hours a day at a 70% reduction would cut it to about $998 — a saving of $645 for a cover that costs a few hundred dollars.
Result: $1,643 for the season, uncovered
Why maintenance dwarfs heat-up
The intuition about pool heating is that the hard part is warming up all that water. Twenty thousand gallons is a lot of water, and raising it twenty degrees does take a genuinely large amount of energy — around 3.5 million BTU, or 35 therms of gas.
But that is a one-off, and it is dwarfed by what comes next. The same pool, held at 82°F against 70°F air with no cover, loses around 30,000 BTU every hour of every day. Over a four-month season that is nearly 90 million BTU — roughly twenty-five times the heat-up.
The reason is simple exposure. A pool presents a large, warm, open surface to the air continuously, and it never stops losing heat through it. The heat-up happens once; the loss happens every hour for the whole season, including all the hours nobody is swimming.
This is why the question "what does it cost to heat my pool" is really a question about surface loss, and why the interventions that matter are the ones that reduce it.
Evaporation is most of the loss
A pool loses heat four ways: evaporation, convection to the air, radiation to the sky, and conduction into the ground. The last is negligible, radiation and convection are modest, and evaporation is roughly 70% of the total.
The reason is the latent heat of vaporisation. Turning a pound of water into vapour absorbs about 1,000 BTU — more than raising that same pound from freezing to boiling. Every pound of water that evaporates from the surface carries away an enormous amount of heat, and a pool losing a quarter of an inch a day from a 512 square foot surface is losing about 80 pounds of water an hour at peak.
Two consequences follow. First, evaporation is driven by the vapour pressure difference between the water and the air, not simply the temperature difference, which is why wind and low humidity increase heat loss dramatically while a still, humid night reduces it. A windy site can lose half again as much as a sheltered one at the same temperature.
Second, and more usefully: anything that stops water leaving the surface stops most of the heat leaving with it. That is what a pool cover does. It is not primarily an insulator; it is a vapour barrier.
The cover is the single best purchase
The Department of Energy puts the saving from a pool cover at 50 to 70% of heating cost. No other intervention in pool heating comes remotely close, and covers cost a few hundred dollars against heaters costing thousands.
- Bubble or solar covers — the cheapest option, essentially heavy-duty bubble wrap floating on the surface. They block evaporation, and the clear ones also transmit some solar gain into the water. They tear, they degrade in UV, and they last two to four seasons — but at a few hundred dollars they pay back within one.
- Automatic covers — a rigid or semi-rigid cover on a motorised reel, deployed at the touch of a button. Costs thousands, and the reason they are worth it is behavioural: a cover that takes five minutes to drag on and off does not get used, and one that takes ten seconds does.
- Liquid covers — a monolayer chemical that spreads across the surface and slows evaporation. Considerably less effective than a physical cover — typically 15 to 30% — and disrupted by wind and swimmers. Useful where a physical cover is impractical.
- When to use it — overnight is the highest-value period, because the air is coldest and the pool-to-air gap widest. Covering only at night, which costs no swimming time at all, captures the majority of the available saving.
- The safety point — a solar or bubble cover is not a safety cover and must never be treated as one. A child or an animal that gets onto it becomes trapped underneath. Safety covers are a separate product, rated to hold weight, and they are what fencing and supervision requirements are about.
Gas against heat pump against solar
The three practical heating technologies differ so much in character that comparing them on running cost alone misses the point.
A gas heater is fast. It produces heat on demand at 82 to 95% efficiency and can lift a pool several degrees in an afternoon, regardless of the weather. That makes it the right choice for a pool used occasionally — a weekend house, or a pool heated for a party — where the ability to warm it up quickly matters more than the cost per BTU. It is also the most expensive to run continuously.
An electric heat pump moves heat from the outside air into the water at a coefficient of performance around 5, which makes it roughly four to five times cheaper to run than resistance heating and typically cheaper than gas. The trade-off is speed and weather dependence: it produces a modest output continuously, warming a pool over days rather than hours, and its capacity falls sharply as the air temperature drops below about 50°F — exactly when you want it most at the shoulders of the season.
Solar pool heating is the outlier and deserves more attention than it gets. Unglazed polymer collectors on a roof, with pool water circulated through them by the existing filter pump, add heat at essentially zero marginal cost. The DOE describes solar pool heating as one of the most cost-effective uses of solar energy in many climates, with payback frequently between two and seven years. The limitations are that it needs roof area roughly half to equal the pool’s surface, it only works when the sun shines, and it cannot hit a target temperature on demand. For a pool used regularly through a summer season it is very hard to beat.
The free adjustments
Before spending anything, two changes cost nothing and are worth more than most equipment decisions.
The first is the thermostat. Heat loss is proportional to the difference between the water and the air, so every degree you lower the target reduces the loss for the whole season. Going from 82°F to 79°F on a pool in 70°F air cuts the temperature gap from 12 degrees to 9 — a 25% reduction in standing loss, and therefore in cost. Most swimmers cannot reliably tell the difference between 80 and 82 degrees, and the American Red Cross suggests 78 to 82°F as comfortable for general swimming.
The second is windbreaks. Since evaporation dominates the loss and wind drives evaporation, sheltering the pool has an outsized effect. A hedge, fence or wall on the prevailing wind side reduces heat loss measurably, and unlike a cover it works without anyone having to remember to deploy it. The shelter needs to be close to the pool and tall enough to affect the air at the water surface — a distant fence does very little.
Between them, a lower setpoint, a windbreak and a cover used overnight can halve a heating bill without changing a single piece of equipment.
What this assumes, and where it stops
Assumptions
- Surface heat loss is proportional to surface area and to the pool-to-air temperature difference, at the coefficient entered.
- The average air temperature applies for the whole 24-hour day across the whole season.
- A cover reduces surface loss by the percentage entered for the hours it is on.
- Heater efficiency is a flat figure. A real heat pump’s COP falls substantially as air temperature drops.
- Solar gain into the pool is not modelled, so a sunny uncovered pool will do somewhat better than this predicts and a shaded one somewhat worse.
Limitations
- The loss coefficient bundles evaporation, convection and radiation into one number. Real evaporation depends on wind speed and humidity, and a windy or arid site can lose half again as much as this suggests.
- Solar gain is excluded. A pool in full sun receives meaningful free heat during the day, which this calculation does not credit.
- Heat pump output and COP both fall as air temperature drops, sharply below about 50°F. Using one flat efficiency overstates a heat pump at the shoulders of the season.
- Using a season-average air temperature smooths over cold snaps, which is when the heater actually works hardest and when capacity, not efficiency, becomes the constraint.
- Nothing here models solar pool heating, which behaves differently from all three heater types and is frequently the most cost-effective option for a regularly used pool.
Common questions
How much does it cost to heat a pool?
For a typical 512 square foot pool held at 82°F through a four-month season with no cover, roughly $1,600 on natural gas or $700 on a heat pump at current US average prices. Maintaining the temperature accounts for well over 90% of that; the initial heat-up is a small one-off by comparison.
How much does a pool cover actually save?
The Department of Energy puts it at 50 to 70% of heating cost, and that matches the physics — evaporation is around 70% of a pool’s heat loss, and a cover is primarily a vapour barrier. Covering overnight alone, which costs no swimming time, captures most of the benefit because the air-to-water gap is widest at night.
Is a pool heat pump cheaper to run than gas?
Usually, and often by half. A heat pump with a COP of 5 moves five units of heat per unit of electricity, where a gas heater delivers 0.84 units per unit of fuel. Even with electricity costing more per BTU than gas, the efficiency gap usually wins. The trade-off is speed — a heat pump warms a pool over days and loses capacity in cold weather, while gas works in an afternoon regardless.
Does pool depth affect heating cost?
Barely, for maintenance. Heat leaves through the surface, so two pools with the same surface area cost nearly the same to hold at temperature regardless of depth. Depth only matters for the initial heat-up, where more water means more energy — and that is the small part of the total.
What temperature should I set my pool to?
Between 78 and 82°F suits most swimming. Every degree lower reduces the gap driving heat out of the surface, so dropping from 82 to 79 against 70°F air cuts the standing loss by about a quarter. Most swimmers cannot reliably distinguish 80 from 82 degrees, which makes the thermostat the cheapest saving available after a cover.
Sources
- Swimming pool heating — options and costs — US Department of Energy
- Swimming pool covers — US Department of Energy
- Solar swimming pool heaters — US Department of Energy
- Electric Power Monthly, Table 5.6.A — average retail price of electricity — US Energy Information Administration
Formula and content last reviewed on .
Results are estimates for information only, not professional advice.
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