A national dollar figure is rarely useful because utility prices, heater efficiency, weather and temperature goals vary. A transparent calculation is better: find the heat needed by the water, translate that heat into purchased energy, then multiply by the local rate.
A national dollar figure is rarely useful because utility prices, heater efficiency, weather and temperature goals vary. A transparent calculation is better: find the heat needed by the water, translate that heat into purchased energy, then multiply by the local rate.
Step 1: calculate the water load
For a 20,000-gallon pool rising 5°F:
20,000 × 8.34 × 5 = 834,000 BTU delivered to the water in an ideal calculation.
Step 2: account for the heating method
For a gas heater, divide the required heat by the unit's thermal efficiency to estimate fuel input. If an example heater is 84% efficient, 834,000 ÷ 0.84 = 992,857 BTU of fuel, or about 9.93 therms. Multiply by the all-in local price per therm.
For electric resistance, divide by 3,412 BTU per kWh. The same water-only load is roughly 244.4 kWh before system losses. Multiply by the applicable electricity rate.
For a heat pump, divide the electric-resistance kWh by the COP at relevant conditions. At a hypothetical COP of 5, the water-only estimate is about 48.9 kWh. Do not assume COP is fixed: it changes with operating conditions, and auxiliary equipment uses electricity too.
Step 3: add what the water-only equation leaves out
The pool loses heat during the cycle. Evaporation can dominate, especially with wind and low humidity. Pump runtime has a cost. Gas bills may include delivery charges or tiered rates; electric bills may include time-of-use pricing and demand rules. Propane is often sold by the gallon, requiring a separate fuel-energy conversion and the appliance efficiency.
Instead of hiding those factors inside a fake precision percentage, calculate a clean baseline and compare it with one actual bill or meter reading. The difference becomes a site-specific operating adjustment.
Build a cost range
Use three cases: covered/mild, typical and uncovered/adverse. Insert the same target temperature but vary expected runtime or delivered performance. This shows whether a purchase decision remains sensible when conditions are less favorable.
The cheapest cycle may not be the best system
Heat pumps can use less purchased energy in suitable weather, but an owner who needs a rapid 20°F rise on a cold day may value gas recovery. Solar can have low operating-energy cost after installation but is tied to sunshine and collector area. Installation cost, service access and expected years of ownership belong beside the per-cycle estimate.
A cost calculator should make every assumption visible. If a result cannot show gallons, rise, efficiency or COP, local rate and whether losses are included, it is not yet a decision-quality number.
Frequently asked questions
How do I calculate gas cost for a pool heater?
Calculate BTUs needed by the water, divide by heater efficiency, divide by 100,000 BTU per therm, then multiply by your all-in price per therm. Add an allowance based on actual heat loss and runtime.
How do I calculate heat-pump electricity cost?
Divide required BTUs by 3,412 and then by COP at relevant conditions. Multiply the resulting kWh by your electricity price, adding pump and auxiliary energy where applicable.
Does heating a pool 1°F cost the same every time?
The water-only energy is consistent for the same gallons, but weather, starting conditions, equipment performance and heat loss can change purchased energy.