Electric resistance
The resistance baseline: a coefficient of performance of 1
In this model, an electric resistance heater turns each kilowatthour at the element into 3,412 BTU of heat. That fixed unit conversion makes the operating arithmetic transparent; under the rates modelled here it is usually costly for full-season pool duty.
Why "100% efficient" is a trap
Resistance heating is often sold on being 100 percent efficient, and the claim is true. It is also the reason it loses. A heat pump does not make heat, it moves heat, so its coefficient of performance is not bounded by 1 — it lands somewhere between four and six on a pool duty. Delivering the same BTU costs four to six times less.
Comparing the two on the same served season: 57,498 kWh on resistance against 10,912 kWh on a heat pump, at the same 15.23 cents per kWh in AZ. That is $8,757 against $1,662 — a difference of $7,095 in this modelled season.
Against gas, the answer depends on your two prices, which is why the calculator below asks for both instead of asserting a winner.
Element size, amperage, and what a degree costs
Resistance sizing is unusually simple: the element rating is the output, and the electrical service is usually the binding constraint.
Heat delivered 51,180 Btu/helement rating times 3,412, exactly
Current drawn 62.5 Aat 240 V — the breaker and feeder have to carry it
Time for the rise 37.5 hourstwelve degrees on this volume, ignoring loss
Cost of one degree $7.13159,815 Btu of water
Cost of the whole rise $85.60before any loss during the heat-up
The electrical service is the real limit
A 15 kW element at 240 volts draws 62.5 amps continuously. Continuous loads are wired at 125 percent, so that is a 80 amp circuit before anything else in the house is considered. Getting enough output to heat a real pool means numbers that most residential panels cannot supply without a service upgrade.
Run it the other way: to deliver the 110,594 BTU per hour maintain load the sizing page computed for the default pool, you need 32.4 kW of element, drawing 135.1 amps at 240 volts. That is not a pool heater, that is a second electrical service.
This is the constraint that decides the question in practice, and it decides it before any argument about running cost. A heat pump delivering the same 110,594 BTU per hour draws roughly a fifth of that current, because it is moving heat rather than making it, and a fifth of the current is the difference between an existing spare breaker and a call to the utility. The running-cost gap is what people argue about; the panel is what actually stops them.
What changes for electric pool heaters in 2028
Electric pool heaters have never had a federal efficiency standard, for the obvious reason that a resistance element has no efficiency to regulate — it is already 1. That changes, and the regulation says so in a sentence that stops mid-thought because the requirement itself is a formula rather than a number:
Gas-fired pool heaters and electric pool heaters manufactured on and after May 30, 2028, shall have an integrated thermal efficiency not less than the following
What it settles here The measure is an integrated one built from certified active electrical power rather than a conversion efficiency, so it is not a number a resistance element can be redesigned to improve — and the rulemaking behind that clause is explicit about what follows.
Rather than infer the consequence, here is how the Department of Energy stated it when assessing the effect on a manufacturer that builds only resistance pool heaters:
the adopted standards result in a minimum efficiency level that is not feasible for electric resistance pool heaters to achieve
What it settles here That is the finding to carry away: from 30 May 2028 a resistance pool heater is not a product that can be newly manufactured for sale in the United States, so if you are choosing between a cheap resistance unit and a heat pump that costs more today, that date is inside the service life of whichever one you buy and only one of the two is still a current product on the far side of it.
The same paragraph draws a line that matters enormously if you were reading this page for a spa rather than a pool:
This small business also manufactures electric resistance spa heaters and commercial electric resistance heating products that would still be allowed to be sold in the United States, even after the compliance date of this final rule.
What it settles here Spa heaters and commercial resistance products sit outside this rule and keep selling, which is the regulatory version of the same conclusion the arithmetic on this page reaches — resistance belongs on small volumes with big, short temperature swings, and the federal standard has drawn its boundary in exactly that place.
None of this makes an existing resistance heater illegal, unsafe or unsupported. Standards of this kind bind manufacture, not ownership or repair, and heating elements remain a stock part. What changes is the market you will be shopping in a few years from now, and the resale position of a pool with a resistance heater bolted to it.
Where resistance is the right answer
There are three cases, and they share a shape: small volume, large temperature swing, short run time.
- Spas. A 400 gallon spa is 3,338 pounds of water. A 15 kW element lifts it twelve degrees in 0.8 hours and the total energy is 11.7 kWh — trivial. Almost every packaged spa on the market uses resistance for exactly this reason.
- Very small plunge pools and cold plunges used intermittently, where the annual energy is small enough that the four-to-one efficiency gap is worth less than the simplicity.
- As a supplement to a heat pump for the handful of hours a season when the air is below the heat pump's cutoff and you want the pool anyway.
What these have in common is that nobody is holding a large surface at temperature for months. The moment that is the duty, the running-cost gap swamps every other consideration.
The third case is the one worth defending in detail, because it is the only place a resistance element earns a spot on a pool that already has a heat pump. The Department of Energy reached the same conclusion when it decided whether electric heat pumps and electric resistance heaters needed separate treatment under the standards:
although heat pump pool heaters perform best when operating within an environment with high air temperature and high air humidity, they are nonetheless capable of operating effectively in cooler climates during the swimming season. Additionally, rare cases in which the ambient temperature is too low for the heat pump pool heater to work effectively could be accommodated through the incorporation of electric resistance backup elements.
What it settles here Read alongside the 50 °F cutoff on the heat pump page, that is the shape of a sensible electric installation: the heat pump carries the season, and a resistance element covers the handful of mornings a year the air is below the point where the compressor can work — an exception measured in hours, not a heater sized to carry the load.
Sizing that backup element is a different calculation from sizing a heater. It is not covering the maintain load for a season, it is recovering a few degrees on the mornings the heat pump sat idle, so the honest input to the calculator above is the volume and the rise you want back, not the seasonal load. An element of a few kilowatts on a circuit that already exists is usually the whole of it.
The one number to keep
3,412 BTU per kilowatthour is a fixed conversion, published by the U.S. Energy Information Administration as a standard conversion factor. It is the anchor for every electric comparison on this site: resistance delivers exactly that, a heat pump delivers that multiplied by its coefficient of performance, and nothing delivers more.