Pool Heating Cost Calculator

Two bills, not one

The energy to heat a pool splits cleanly into a stock and a flow. The stock is the heat you put into the water once, to move it from where it is to where you want it. The flow is the heat that leaves the surface every hour thereafter, which you must keep replacing for as long as you want the pool warm.

The stock is the easy half, and it is exact. Water weighs 8.34 pounds per US gallon and a Btu is defined as the energy that raises one pound of water by one degree Fahrenheit, so a 20,000 gallon pool needs 166,800 Btu per degree. Ten degrees is 1.668 million Btu — about seventeen therms of gas at the burner, before efficiency.

The flow is the harder half, and it is an estimate. A warm pool loses heat four ways: evaporation, which usually dominates and is driven by wind and humidity; convection to the air; radiation to the night sky; and conduction into the ground, which is small. Engineering practice bundles the first three into one coefficient of roughly 5 Btu per hour per square foot per degree of water-to-air difference, adjusted up or down for exposure. That is the number this calculator uses, and it is why surface area rather than volume drives the monthly bill. A deep pool and a shallow pool of the same footprint cost nearly the same to hold at temperature; only the initial heat-up differs.

Why efficiency, COP and price all sit in different places

Every path in this calculator delivers the same Btu to the water. What differs is what you have to buy to produce those Btu, and that is where the three conversion factors come in.

Combustion efficiency applies to gas and propane, and it divides. A heater rated 400,000 Btu/hr input at 84% efficiency delivers 336,000 Btu/hr to the water; to put 1,668,000 Btu into the pool you must burn 1,985,714 Btu of gas. That is why the fuel term is Q ÷ efficiency and not Q × efficiency.

COP applies to heat pumps and also divides, but it is greater than one. A heat pump does not make heat, it moves heat from the air into the water, so a COP of 5.5 means each kilowatt-hour of electricity delivers 5.5 kWh of heat. The electricity needed is Q ÷ (3,412.14 × COP). The catch is that COP is quoted at a rating condition — typically warm air and cool water — and it falls as air temperature drops, which is exactly when you most want the heat.

Electric resistance has neither: every kilowatt-hour becomes 3,412.14 Btu of heat, no more and no less. It is the simplest path and, at typical residential rates, the most expensive per delivered Btu.

The cover does not change any of that arithmetic; it changes the loss term. Covering the pool for part of the day cuts the loss during those hours by the reduction percentage, which the calculator converts into an effective number of uncovered hours. Fourteen hours under a cover that cuts loss by 60% is arithmetically the same as 5.6 uncovered hours, so the pool behaves like a 15.6-hour-a-day pool instead of a 24-hour one.

Reading the result, and sizing a heater from it

Start with the hours-to-heat figure, because it is the sizing check. A common rule of thumb sizes a pool heater to raise the water 1 to 2 °F per hour on an uncovered pool, which for the example above means 166,800 to 333,600 Btu/hr delivered. If the calculator tells you your heater needs two days to make a ten-degree rise, it is undersized for on-demand use — workable if you leave the pool at temperature all season, frustrating if you heat it for weekends.

Then read the monthly figure against the fuel comparison table. The ranking between fuels is not fixed; it is entirely a function of the three prices you entered. A heat pump beats natural gas when its cost per delivered Btu is the lower of the two, and setting the two expressions equal gives the break-even electricity rate directly: rate = gas price × 3,412.14 × COP ÷ (efficiency × 100,000). At $1.40 a therm, 84% efficiency and COP 5.5 that is 1.40 × 18,766.77 ÷ 84,000 = $0.313 per kWh. So at the $0.17 in the defaults the heat pump is the cheaper path here, and above about $0.31 it would not be. Change the prices to your own before drawing a conclusion.

The cover saving is the single largest lever on this page for most pools, and it does not depend on the heater at all. It is proportional to the hours covered and to the reduction percentage, so a cover used only overnight in a climate where the water-air gap is largest overnight captures more than the hour count suggests.

Finally, remember what the model omits: solar gain. A pool in full sun collects a real quantity of heat during the day, which is why unheated pools in summer run above air temperature and why a clear bubble cover both retains heat and admits some solar. This calculator gives you the heating side of the ledger only, so treat the monthly figure as an upper bound for a sunny site.

Cost of one million delivered Btu by fuel path

The ranking follows entirely from the three prices at the top of the caption. Enter your own rates in the calculator, since a region with cheap gas and expensive electricity reverses the bottom two rows.

Assumptions and limits worth knowing

• No solar gain. Daytime sun heats a pool for free, sometimes by several degrees. This model counts only losses, so it overstates the cost on a sunny, sheltered site.
• One loss coefficient for all conditions. Evaporation depends on humidity as well as wind, so a dry desert night loses far more than a humid coastal one at the same air temperature. The exposure selector is a coarse proxy for that.
• Average air temperature, not daytime high. Entering an afternoon temperature will understate the monthly cost substantially, because the water-air gap is widest at 4 a.m.
• Heat pump COP is a single number here. Real output and COP both fall with air temperature, and most units cut out somewhere near 50 °F air. A single COP is only valid over a narrow band of conditions.
• Thirty-day months. The monthly figures are exactly thirty daily figures, so scale them if you are budgeting a 31-day month or a partial season.
• The heat-up assumes continuous run. If the heater cycles on a thermostat or shares a gas meter with the house, the elapsed time will be longer than the calculated run hours.

Where pool heating sits among household energy decisions

A heated pool is often the largest single energy load a home carries, larger than space heating in mild climates, and it is one of the few loads where a passive intervention — a cover — routinely cuts the bill by more than half. That is a return no equipment upgrade matches, which is why it is worth modelling before the heater choice.

The comparison logic on this page is the same logic that governs any appliance decision: separate the energy the appliance must deliver from the price of buying that energy, then let the prices decide. The appliance repair vs replace calculator applies it to a failing machine, and the appliance energy cost calculator does the per-device version of the kilowatt-hour arithmetic used here.

If you are budgeting a whole outdoor season, the pool is one line among several. The UV index sunburn time calculator covers the safety side of the same afternoons, and if the pool sits in a yard you are also planting or trimming, the square foot garden plant spacing calculator and the baseboard and trim linear feet calculator handle the other quantities that come with the property.