Gas Heater vs. Heat Pump: Choosing (and Budgeting for) Pool Heat
Adding heat to a pool changes the math on almost everything — when you can swim, how long the season lasts, and how much the pool costs to run. The two most common heating technologies, gas heaters and electric heat pumps, sit on opposite ends of a trade-off: one heats quickly and costs more per hour to run, the other heats slowly and costs much less.
How a gas heater works
A gas heater burns natural gas or propane to directly heat water passing through it, similar in principle to a tankless water heater. Because it's a direct combustion process, its efficiency — the percentage of the fuel's energy that actually ends up as heat in the water — tops out below 100%, with modern gas pool heaters commonly rated somewhere in the 78-95% range. Gas heaters typically have a high heat output (measured in BTU per hour), which is why they can raise a pool's temperature relatively quickly, often overnight or even within a few hours for a smaller pool.
How a heat pump works
A heat pump doesn't generate heat by burning fuel; instead, it uses electricity to run a compressor that extracts existing heat from the surrounding air and transfers it into the pool water, the same basic principle behind a refrigerator running in reverse. Because it's moving heat rather than creating it from scratch, a heat pump can deliver more energy in heat output than it consumes in electricity — this ratio is called its coefficient of performance (COP), and residential pool heat pumps commonly report a COP somewhere around 4 to 7 depending on air temperature and model. A COP of 5, for instance, means the unit delivers roughly five units of heat energy for every one unit of electrical energy it consumes.
Why heat pumps are slower
The trade-off for that efficiency is heat output: heat pumps generally have a lower total BTU/hr output than a comparably priced gas heater, so heating the same pool by the same number of degrees takes longer. A heat pump is a poor fit if you want to heat a cold pool quickly for a single weekend event, but a strong fit if you're keeping the pool at a steady temperature all season, since the ongoing running cost is so much lower.
Estimating the cost to heat your pool
Regardless of technology, the physics of heating water is the same: it takes 1 BTU to raise 1 pound of water by 1°F, and a gallon of water weighs about 8.34 pounds. So the total heat energy needed is your pool's gallons × 8.34 × the number of degrees you want to raise the temperature. A 15,000-gallon pool going from 70°F to 85°F (a 15-degree rise) needs roughly 15,000 × 8.34 × 15 ≈ 1,876,500 BTU of delivered heat, regardless of which heater does the delivering.
From there, divide that total by how fast your heater actually delivers usable heat — its rated input multiplied by its efficiency (or, for a heat pump, its COP) — to get the hours required, then multiply the heater's input rate by that run time to find the fuel or electricity consumed, and multiply that by your local price per therm or per kWh for the cost.
A rough comparison
Run the numbers on that same 15,000-gallon, 15-degree example and the difference in operating cost becomes clear: a mid-efficiency gas heater might take under ten hours and cost somewhere in the neighborhood of $35-40 in gas at typical prices, while a heat pump with a COP around 5 might take closer to 22 hours but cost roughly half as much in electricity, because it's moving far more energy than it consumes. The gas heater wins decisively on speed; the heat pump wins decisively on cost per degree.
Cutting the cost either way: a cover
Whichever heater you choose, a solar or insulated pool cover is one of the most effective ways to reduce ongoing heating cost, because it cuts evaporation — the single biggest source of heat loss from an outdoor pool — and holds warmth overnight. A cover doesn't change the one-time heat-up calculation, but it substantially reduces how much top-up heating you need afterward to hold that temperature.
Plug your pool's volume, current and target temperature, and your heater's specs into the Pool Heater Running-Cost Estimator to see the run time and cost for your specific setup, gas or electric.