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 gas heater at 82% efficiency with a 250,000 BTU/hr input, running at roughly $1.35 per therm, comes out to about 9.15 hours and $30.89 for that one-time heat-up. A heat pump with a COP around 5 (a 5 kW input rate, at $0.16 per kWh) takes noticeably longer — about 22 hours — but costs about $17.60, roughly 40% less than the gas heater for the identical temperature rise, because it's moving far more heat energy than it consumes in electricity. The gas heater wins decisively on speed; the heat pump wins on cost, and the gap in this example only widens for ongoing, season-long operation rather than a single heat-up. A full breakdown of what that gap looks like over an entire season, not just once, is in What It Really Costs to Heat a Pool for a Season.

Sizing matters as much as technology

Comparing gas and heat pump technology in the abstract only tells half the story — an undersized unit of either type will disappoint you regardless of which one you pick. A heater sized too small for the pool's volume and the local climate's heat loss will struggle to ever reach the target temperature, especially overnight or during a cold snap, and will run far more total hours (and cost more) trying. Sizing is typically driven by the same BTU-needed math the running-cost calculation uses, extended to account for a pool's surface area and typical wind exposure, since a larger, more exposed pool loses heat faster and needs more sustained output to hold a temperature, not just to reach it once. When in doubt, a unit sized toward the larger end of a manufacturer's recommended range for your pool's volume tends to age better than one sized to the bare minimum, since it won't need to run at maximum output as constantly to keep up.

Hybrid approaches: not always an either/or choice

Some pool owners split the difference rather than picking one technology exclusively — running a gas heater for a fast initial heat-up ahead of a specific event, then relying on a heat pump (or simply the sun and a cover) to hold that temperature afterward at a much lower ongoing cost. This isn't practical for every setup, since it means owning and maintaining two heating systems, but for a pool that's heated occasionally rather than kept at a steady temperature all season, it can capture the speed advantage of gas without paying gas prices for the entire time the pool stays warm. Whether this makes sense financially depends heavily on how often the pool actually needs a fast heat-up versus how long it holds temperature afterward — worth running the numbers on your own usage pattern rather than assuming either extreme. A pool heated for a handful of weekends a summer leans toward gas alone being simpler; a pool held at a swimming temperature for months at a stretch leans hard toward a heat pump (or a hybrid) purely on the operating-cost side of the comparison.

Maintenance differences worth knowing about upfront

The two technologies also differ in what ongoing maintenance looks like. Gas heaters have a combustion process to maintain — a burner, ignition system, and venting that need periodic inspection, particularly before each heating season, since a fouled burner or blocked vent is both an efficiency problem and, for gas equipment specifically, a potential safety issue worth having a qualified technician check rather than troubleshooting blindly. Heat pumps have no combustion involved, but rely on a refrigerant cycle and defrost cycle in colder ambient conditions, and their efficiency (that COP figure driving much of their cost advantage) tends to drop as outdoor air temperature falls, which is part of why heat pumps are generally better suited to climates that don't see extended stretches of very cold air even when the pool itself needs to stay warm.

Where a plain electric resistance heater fits in

There's a third, less common option worth a mention: a straightforward electric resistance heater, which works like an oversized version of a household water heater's element rather than either burning fuel or moving heat from the air. Its efficiency is close to 100% — nearly all the electricity it draws becomes heat in the water, with very little lost — which sounds good until you compare it to a heat pump's effective 400-700% "efficiency" (its COP expressed as a percentage), since a heat pump is moving existing heat rather than converting electricity directly. A resistance heater ends up costing roughly four to seven times as much to deliver the same amount of heat as a heat pump under typical conditions, which is why it's rarely chosen as a primary pool heater today and shows up more often as a small backup or spa heater than as the main system for an entire pool, where the smaller volume of water being heated makes the higher per-degree cost less painful in absolute dollar terms. Its one practical advantage is simplicity — no refrigerant cycle, no outdoor-air dependency, and it works identically regardless of ambient temperature, which is occasionally worth the cost premium for a small spa that needs reliable heat in cold weather a heat pump would struggle with.

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, which matters more over a season than the one-time number by itself suggests.

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.