How much does it cost to heat a pool?

It depends on whether you mean the one-time heat-up or the ongoing maintenance. For a standard 20,000-gal pool, raising temperature 15 °F costs about $46 on gas one time. Holding that temperature uncovered against the standing loss costs about $1,100 a month on gas at default rates. The maintenance number is what you actually pay; the heat-up is what most calculators incorrectly headline.

Why is my pool heating bill so high?

Because your pool loses heat to its surface 24/7 — most of it as evaporation — and the heater is replacing that loss continuously. Pools don't insulate; the top surface is open to the sky. Even at default rates, the standing loss can be over 80,000 BTU per hour on a standard uncovered pool. Block evaporation with a cover and the loss drops about 65 %.

Is a pool cover worth it?

Yes — by a wide margin. A cover cuts the standing loss about 65 %, which on a standard pool at default gas rates saves about $719 a month. A $200 cover earns itself back in about 8 days of heating, then keeps paying. Where electricity is expensive, the payback drops to days. It's the single highest-ROI purchase in pool ownership.

Gas or heat pump — which is cheaper to run?

Heat pump, in warm weather, by roughly 2× — it delivers about five units of heat per unit of electricity instead of burning fuel for one. But the COP sags as air cools, dropping toward 3.8 around 55 °F and lower below that. In shoulder seasons the gap narrows; in cold weather a heat pump can even cost more to run than gas. Right for steady warm-season heating; gas for fast or cold.

How much does it cost to heat a pool to 90 °F?

Each extra degree above ambient adds about $74 per month on gas for a standard uncovered pool at default rates. Going from 82 °F to 90 °F is roughly 8 extra degrees, or about $590 a month more than the 82 °F baseline. The cheapest extra degree is the one your cover protects.

Is electric resistance heating ever worth it for a pool?

Almost never. Resistance heat is about 5× a heat pump's running cost for the identical heat delivered — there's no COP multiplier because resistance just turns electricity directly into heat at 100 % efficiency. Maybe for a spa where short cycle times outweigh per-BTU cost; almost never for a full pool.

Why does my heat pump cost more in spring and fall?

Two things happen at once. The COP drops in colder air — the rated 5 in warm weather sags toward 3.8 around 55 °F — so each delivered BTU costs more electricity. And the loss to ambient rises because the water-air gap is bigger. But what specifically erodes the heat pump's advantage over gas is its COP falling, not the higher loss. The loss increase hits gas and heat pump equally and cancels in their ratio.

How accurate is this estimate?

Honestly: doubly an estimate. The underlying heat loss varies 2–3× with humidity, wind, and surface conditions. And fuel prices vary 2–4× by region and swing year-to-year. That's why every price is your input, not ours, and every output is a range. Use these figures to compare strategies — cover vs no cover, drop 3 °F, switch fuels — not to forecast your bill to the dollar.

Pool Heating Cost Calculator

The $46 figure most calculators quote is the cost you pay once. The number that matters is what it costs to keep the pool warm — and on an uncovered pool, that runs over a thousand dollars a month. This calculator shows both costs honestly, at your fuel rates, and surfaces the lever that beats every heater upgrade: a cover.

Hook

“It costs about $46 to heat your pool.”

That's the figure most calculators give you — the one-time cost to warm the water up — and it's almost irrelevant. The number that matters is what it costs to keep it warm. For an uncovered pool, that can run over a thousand dollars a month, because your pool is evaporating its heat into the sky around the clock. The heat-up is a rounding error next to the standing loss.

Promise

This calculator separates the two costs honestly — the one-time heat-up and the ongoing maintenance — across natural gas, propane, heat pump, and electric resistance, using your fuel rates (not a hardcoded guess). It shows the gas-vs-heat-pump per-BTU reality, what each extra degree of warmth costs you per month, and the one number that makes everyone reconsider: how fast a pool cover pays for itself. For the time half of the trade-off (how many hours that heat-up takes), see the heat-up-time calculator. Every figure derived on the page.

Here's the deal: heating cost is just energy × price ÷ efficiency. The energy is mostly the heat your pool loses at the surface — dominated by evaporation — replaced hour after hour. So the cost question is really a heat-loss question wearing a dollar sign, which means the cheapest path to a warm pool usually isn't a better heater. It's a cover.

The calculator

Enter the same pool geometry the heater-sizing page uses — volume + surface area, water + air temp, wind, cover — plus your local fuel rates. The result panel shows the one-time heat-up cost and the monthly maintenance cost across all four fuels, the cover-payback headline regardless of toggle, and the per-degree slope so you can see what each extra °F is worth.

Pool volume

Sets the heat-up energy.

Surface area (drives the monthly bill)

Length (ft)

Width (ft)

Blank → estimated from gallons at 5ft depth (an “estimate” badge will show).

Current water (°F)

Target water (°F)

Average air (°F)

Wind exposure

Cover

The result panel shows both with-cover and without-cover side by side regardless of this toggle — this just picks which one we headline.

Your local rates (editable placeholders)

Replace these with what your bill actually shows — fuel rates vary 2–4× by region and they swing year-to-year. The defaults are research anchors, not predictions.

Gas ($/therm)

Propane ($/gal)

Electricity ($/kWh)

Heat pump COP

Rated COPs are usually ~5 in warm air; drops to ~3–4 in shoulder seasons. Leave at 5 to auto-adjust for your air temp.

Cover price ($)

Used for the payback calc.

Don't know your gallons? Pool volume calculator — two minutes, any shape, deep-links back here with ?gal= prefilled.

The two costs, and why everyone quotes the wrong one

There are two completely different “cost to heat” numbers, and the pool internet almost always gives you the small one. The first is the cost to warm the water up — heat 20,000 gallons by fifteen degrees, that's about $46 on gas, one time. The second is the cost to keep it warm — replace the heat your pool sheds into the air every hour, around the clock.

That second number, for an uncovered pool, is over a thousand dollars a month. The heat-up is the rounding error; the standing loss is the bill. If a calculator told you “$46 to heat your pool” and stopped, it answered the question nobody's actually paying.

Almost every cost-to-heat calculator on the web quotes the left bar — the one-time energy to warm the water — and stops there. The right bar is what shows up on your bill, month after month: the energy to hold that water against evaporation, wind, and conduction. Quoting only the heat-up is the dollar equivalent of quoting only the asking price of a house and ignoring the mortgage.

A cover pays for itself in about a week

If most of your heating bill is replacing evaporated heat, then a cover — which stops evaporation — isn't a nice-to-have. It's the highest-return purchase in pool ownership. On our standard pool, a cover cuts the monthly gas maintenance from about $1,100 to about $390. That's roughly $720 a month back in your pocket.

A $200 cover earns itself back in about eight days of heating, then keeps paying every day after. The math scales with your fuel price — where electricity is expensive (Europe, much of California) the payback is in days, not weeks.

Evaporation is by far the largest path heat leaves a pool, and a cover blocks it. The difference between the two bars is what a cover saves you in a month — divide your cover's price by the daily share of that saving, and you have your payback. Even cheap solar bubble covers usually pay back inside a season; this is the single biggest ROI lever on the page.

Gas vs heat pump vs the rest — the per-BTU truth

Every fuel delivers the same BTU; they just charge differently for it. A heat pump is the cheapest to run — it doesn't make heat, it moves it, so it delivers four to six units of warmth per unit of electricity — but it's slow, and that efficiency sags in cold air. Gas costs more per BTU but heats fast and doesn't care about the weather. Propane is gas's pricier rural cousin. And electric resistance is the one to avoid for a pool: about five times a heat pump's running cost for the identical heat.

A heat pump's 2× running-cost edge is a warm-weather fact. As the shoulder seasons start and outside air drops toward 55°F, the heat pump's coefficient of performance falls — from a rated ~5 toward ~3.8 — and the gap to gas narrows. Colder air costs more to heat on every fuel; what specifically erodes the heat pump's advantage is its COP falling, not the higher loss (both fuels pay the higher loss). No fuel is universally right — the user's climate, usage pattern, and rates decide.

At default fuel rates and warm air, heat pump is cheapest to run, gas is roughly 2× more, propane and electric resistance roughly 4–5× more. The order is fragile, though: in regions with cheap natural gas or expensive electricity, gas can match or beat heat pump. The COP-sag band on the heat-pump bar is honest — colder air costs more to heat on every fuel, but what specifically erodes the heat pump's advantage is its COP falling, not the higher loss (both fuels pay the higher loss). Equipment cost is a separate trade-off the running bars don't show.

What does each degree cost?

The difference between an 82°F pool and an 88°F pool isn't six degrees of comfort — it's a line item. Every degree warmer widens the gap between your water and the air, so every degree leaks faster and costs more to replace.

On our standard uncovered pool, each extra degree adds roughly $74 a month on gas. Six degrees warmer is about $440 a month — which reframes “let's bump it up a little” as an actual budget decision. After the cover, this is the single biggest lever the user controls.

Heat loss is linear in the gap between water and air, so cost is too. The pool-temperature slider is the single biggest lever after the cover: dropping the target by 3°F saves about $220/month on this pool at default gas rates. Your slope is steeper if you have more surface area, more wind exposure, or higher fuel prices — all of which the calculator picks up from your inputs.

Where the numbers come from

Heating cost is just energy × price ÷ efficiency. The engine in five steps — same thermal physics as the sizing page, plus one new unit-conversion layer.

Step 1 · the BTU come from the sizing engine
Heat-up energy in BTU and standing loss in BTU/hr both come from the same heater-sizing engine — first-principles for the heat-up half (mass × specific heat × ΔT) and a typical-conditions estimate for the surface loss. We importthose functions; we don't recompute the physics. Drift here would also trip the heater-sizing page's tests.
Step 2 · fuel energy contents (standard)
1 therm of natural gas = 100,000 BTU. 1 US gallon of propane = 91,500 BTU. 1 kWh of electricity = 3,412 BTU. These are dimensional facts, not estimates.
Step 3 · efficiencies & COP (typical, editable)
Modern gas/propane pool heaters land around 82 % efficient (some hit 84–95 %; 82 % is a safe typical anchor). Electric resistance is 100 % by definition. Heat pumps don't make heat — they move it — so their COP sits around 5 in warm air. Each of these is labelled typical and editable in the calculator.
Step 4 · the dollar formula
Gas: BTU ÷ efficiency ÷ 100,000 × $/therm. Propane: BTU ÷ efficiency ÷ 91,500 × $/gal. Heat pump: BTU ÷ COP ÷ 3,412 × $/kWh. Resistance: BTU ÷ 3,412 × $/kWh. Same shape, four different denominators.
Step 5 · sanity check (E1 + E2)
20,000 gal, 800 ft², 70°F → 85°F, air 70°F, average wind, uncovered. E1 heat-up energy = 2,503,500 BTU → at $1.50/therm and 82 % efficiency, ≈ $46 one-time. E2 standing loss = 84,000 BTU/hr → 60M BTU/month → ≈ $1,106/month on gas. The monthly maintenance dwarfs the heat-up by ≈ 24× — the §4.1 wedge.

Seven worked examples

All BTU consume the asserted lib/thermal/ engine; all dollars use the page's default rates ($1.50/therm gas, $3.00/gal propane, $0.16/kWh, COP 5). Your rates change the numbers; the relationships hold.

E1 — The one-time heat-up cost (the number everyone quotes)

20,000 gal, raise 15 °F → 2.50M BTU. Gas ~$46, propane ~$100, heat pump ~$23, resistance ~$117 — once.

Takeaway:this is the figure most calculators headline. Real, but it's the cost you pay once. Keep reading for the one you pay every month.

E2 — The monthly maintenance cost (the real number)

800 ft², water 85 °F, air 70 °F, average wind, uncovered, held continuously. Standing loss = 84,000 BTU/hr → 60.5M BTU/month. Gas ~$1,106/month, heat pump ~$567/month.

Takeaway:this is what it actually costs to keep an uncovered pool warm. Quoting only the E1 number is the field's core mislead — the pool is evaporating heat into the sky 24/7.

E3 — The cover payback (the headline ROI)

Same pool, covered: loss drops to 29,400 BTU/hr → ~$387/month on gas. Cover saves ~$719/month; a $200 cover pays back in ~8.3 days.

Takeaway: the highest-return purchase in pool ownership — pays for itself in about a week, then saves $700+ a month every month after.

E4 — Gas vs heat pump head-to-head (the equipment decision)

Monthly maintenance, uncovered: gas ~$1,106 vs heat pump ~$567. Heat pump runs ~2.0× cheaper — in warm weather. The shoulder-season gap narrows (see E7).

Takeaway: heat pump for steady warm-season heating; gas for fast or cold.

E5 — What each degree costs (the 88-vs-82 decision)

Standard uncovered pool, raising the target 1 °F adds ~$74/month on gas. Going from 82 °F to 88 °F is ~$443/month more.

Takeaway:“Let's bump it up a few degrees” is a real budget line. The cheapest degree is the one you cover.

E6 — Propane (the rural reality)

Same heat-up as E1 on propane at $3.00/gal: ~$100 one-time, and proportionally higher monthly maintenance than gas.

Takeaway:if you're on propane, the cover math (E3) matters even more — the per-BTU cost is the highest of any combustion fuel.

E7 — Heat pump in the shoulder season (the COP-sag honesty)

Same pool, air dropped from 70 °F to 55 °F. Loss roughly doubles (121M BTU/month) and the heat-pump COP sags from 5 to ~3.8. Warm-air gas/HP ratio = 2.0×; cold-air ratio narrows to 1.5×.

Takeaway:colder air costs more to heat on every fuel — but what specifically erodes the heat pump's advantage is its COP falling, not the higher loss. The loss rise hits both fuels equally and cancels in their ratio. The COP-sag is the real story.

Reference tables

T1 · Monthly maintenance cost by pool size × fuel

ESTIMATE · uncovered, water 85 °F, air 70 °F, average wind, default rates ($1.50/therm gas, $3.00/gal propane, $0.16/kWh, COP 5). Each cell shown as a ±10 % range; your local rates change the number.

Pool	Gas	Propane	Heat pump	Resistance
Small (12,000 gal)	$622–$761	$1,360–$1,663	$319–$390	$1,595–$1,950
Standard (20,000 gal)	$996–$1,217	$2,176–$2,660	$510–$624	$2,552–$3,120
Large (30,000 gal)	$1,369–$1,673	$2,993–$3,658	$702–$858	$3,510–$4,290

T2 · Per-million-BTU running cost by fuel

Rate-dependent · default rates above. Heat-pump figure is COP 5 (warm air) — drops in cold air (F8). Use as a per-BTU comparison; absolute dollars scale with your bill.

Fuel	Cost / 1M BTU delivered	Note
Heat pump	$9.38	Cheapest in warm air; COP sags ~3–4 in cold.
Natural gas	$18.29	Fast and weather-independent.
Propane	$39.98	Gas's rural cousin — same BTU, higher price.
Electric resistance	$46.89	Rarely sensible for a pool — ~5× a heat pump.

T3 · Cover savings & payback by pool size

ESTIMATE · uncovered → covered monthly gas saving, and the payback days for a $200 cover at default rates. The single most actionable number on the page.

Pool	Uncovered gas	Covered gas	Saving / mo	Payback ($200)
Small (12,000 gal)	$691	$242	$449	≈ 13.3 days
Standard (20,000 gal)	$1,106	$387	$719	≈ 8.3 days
Large (30,000 gal)	$1,521	$532	$989	≈ 6.1 days

Tables released CC BY 4.0. Energy figures are exact (the lib/thermal/ engine the heater-sizing page asserts); dollar figures are estimates at editable rates.

Methodology & sources

Cost = energy ÷ efficiency × price, applied to first-principles physics. The energy comes entirely from the heater-sizing engine (lib/thermal/) — this page imports heatUpEnergyBtu and surfaceLossBtuHrrather than recomputing physics. The shared-engine identity is asserted in tests: this page's BTU figures equal the sizing page's by construction.

Fuel energy contents are standard: 1 therm = 100,000 BTU, 1 US gal propane = 91,500 BTU, 1 kWh = 3,412 BTU. Modern gas/propane pool-heater efficiency is taken as 82 % — a labelled, editable typical (some hit 84–95 %). Heat-pump COP defaults to 5 in warm air and is auto-adjusted toward 3.8 as air temperature drops to 55 °F (the F8 honesty point).

We state plainly: cost is doubly an estimate— heat-loss varies 2–3× with humidity and wind, fuel prices vary 2–4× by region and swing year-to-year. That's why every price on this page is your input and every output is a range, never a hardcoded “your cost is $X.” And we don't advocate a fuel or sell a heater — the per-BTU economics are presented even-handedly across gas, propane, heat pump, and resistance.

The cover-payback finding (E3) is the page's headline: the cover is the highest-return purchase in pool ownership, grounded in the evaporation dominance the sizing page established. Reference tables T1/T2/T3 are released under CC BY 4.0 — energy columns exact, dollar columns estimates at editable rates.

Frequently asked questions

How much does it cost to heat a pool?: It depends on whether you mean the one-time heat-up or the ongoing maintenance. For a standard 20,000-gal pool, raising temperature 15 °F costs about $46 on gas one time. Holding that temperature uncovered against the standing loss costs about $1,100 a month on gas at default rates. The maintenance number is what you actually pay; the heat-up is what most calculators incorrectly headline.
Why is my pool heating bill so high?: Because your pool loses heat to its surface 24/7 — most of it as evaporation — and the heater is replacing that loss continuously. Pools don't insulate; the top surface is open to the sky. Even at default rates, the standing loss can be over 80,000 BTU per hour on a standard uncovered pool. Block evaporation with a cover and the loss drops about 65 %.
Is a pool cover worth it?: Yes — by a wide margin. A cover cuts the standing loss about 65 %, which on a standard pool at default gas rates saves about $719 a month. A $200 cover earns itself back in about 8 daysof heating, then keeps paying. Where electricity is expensive, the payback drops to days. It's the single highest-ROI purchase in pool ownership.
Gas or heat pump — which is cheaper to run?: Heat pump, in warm weather, by roughly 2× — it delivers about five units of heat per unit of electricity instead of burning fuel for one. But the COP sags as air cools, dropping toward 3.8 around 55 °F and lower below that. In shoulder seasons the gap narrows; in cold weather a heat pump can even cost more to run than gas. Right for steady warm-season heating; gas for fast or cold.
How much does it cost to heat a pool to 90 °F?: Each extra degree above ambient adds about $74 per month on gas for a standard uncovered pool at default rates. Going from 82 °F to 90 °F is roughly 8 extra degrees, or about $590 a month more than the 82 °F baseline. The cheapest extra degree is the one your cover protects.
Is electric resistance heating ever worth it for a pool?: Almost never. Resistance heat is about 5×a heat pump's running cost for the identical heat delivered — there's no COP multiplier because resistance just turns electricity directly into heat at 100 % efficiency. Maybe for a spa where short cycle times outweigh per-BTU cost; almost never for a full pool.
Why does my heat pump cost more in spring and fall?: Two things happen at once. The COP drops in colder air — the rated 5 in warm weather sags toward 3.8 around 55 °F — so each delivered BTU costs more electricity. And the loss to ambient rises because the water-air gap is bigger. But what specifically erodes the heat pump's advantage over gas is its COP falling, not the higher loss. The loss increase hits gas and heat pump equally and cancels in their ratio.
How accurate is this estimate?: Honestly: doubly an estimate. The underlying heat loss varies 2–3× with humidity, wind, and surface conditions. And fuel prices vary 2–4× by region and swing year-to-year. That's why every price is your input, not ours, and every output is a range. Use these figures to compare strategies — cover vs no cover, drop 3 °F, switch fuels — not to forecast your bill to the dollar.

Related calculators

Next in Pool Heating: Pool Evaporation Calculator.

Related across clusters: Pool Pump Cost Calculator.

All Pool Heating calculators: browse the hub.