The physics, briefly
Raising one gallon of water one degree Fahrenheit takes 8.34 BTU — so a 15,000-gallon pool needs about 125,000 BTU per degree, no matter what makes the heat. The heater choice only changes what that BTU costs: a therm of gas holds 100,000 BTU (times ~82% burner efficiency), a kilowatt-hour holds 3,412 BTU (times the heat pump's multiplier, or times one for a resistance heater).
BTU needed = gallons × 8.34 × °F rise · heater output = BTU needed ÷ heat-up hours
Holding temperature is a different bill: the pool constantly loses heat to the air, mostly through evaporation at the surface. The monthly figure uses the standard planning rule — roughly 5 BTU per hour per square foot per degree above air temperature for an uncovered pool, about a third of that covered — applied to your temperature rise around the clock.
How to spend less
In order of payback: cover the pool (biggest lever, cheapest fix), block wind (a hedge or fence can halve losses on exposed lots), heat to the temperature you actually swim at rather than a round number — each degree of target costs the same to hold, forever — and only then shop heaters. Your local season math matters too: the season lookup shows the free-swimming core a heater would be extending.
Frequently asked questions
Gas heater or heat pump — which is cheaper?
Heat pumps are almost always cheaper to RUN (they move heat instead of making it, delivering 4–6 units of heat per unit of electricity), while gas is cheaper to buy and heats much faster. Rule of thumb: heat pump for holding a steady temperature all season, gas for weekend-cottage pools you heat up on demand — and gas keeps working in cold air, where heat-pump output sags.
What size heater do I need?
Size to your surface area and how fast you want heat. The calculator shows the output needed for your chosen heat-up time; as a floor, most installers want ~50,000 BTU/hr per 10,000 gallons for gas, and the biggest heat pump the budget allows (oversizing a heat pump costs little to run — it just cycles less).
How much does a cover change the bill?
More than the heater choice does. Evaporation is most of a pool's heat loss, and a simple solar cover cuts total losses by roughly two-thirds. The monthly figures in this calculator swing about 3× between covered and uncovered — that's the real answer to "why is my gas bill huge?"
How exact are these costs?
They're planning estimates. Real losses depend on wind, humidity, night temperatures, and how often the cover is actually on. The physics constants are solid (8.34 BTU per gallon-degree; ~100,000 BTU per therm); the loss model uses standard industry rules of thumb. Edit the energy prices to match your utility bill — defaults are national ballpark figures.