Why Your Heat Pump Bill Is So High: The Auxiliary Heat Trap
Heat pumps can deliver up to three times more heat than the electricity they use. Backup resistance strips deliver one-to-one, and in some homes they end up doing much of the work.
Why would the most efficient heater most homes can buy produce a winter electric bill that looks like it came from a space heater? Often because, for part of the winter, that is effectively what's running.
Nearly every central heat pump hides a second heating system inside the air handler: coils of electric resistance wire, known as backup, auxiliary or "strip" heat. They exist for good reasons. But when they switch on more than they should, they quietly erase the efficiency the homeowner paid for, and a field study from the Energy Department's National Renewable Energy Laboratory shows how often that happens in real houses.
Why is my electric bill so high with a heat pump?
Start with what a heat pump is supposed to do. It moves heat rather than making it, which is how it can deliver "up to three times more heat energy to a home than the electrical energy it consumes," according to the Energy Department's Energy Saver guidance. In a Northeast study the department cites, replacing electric resistance heaters with air-source heat pumps in colder regions saved about 3,000 kWh, or $459, a year.
Resistance strips have no such trick. Every unit of electricity becomes one unit of heat, no more. The Energy Department's operating guide says backup systems such as electric resistance or strip heaters "are usually more expensive to operate because they are less efficient." So the single biggest question behind a shocking bill is usually not whether the heat pump is working, but how much of the heating it is really doing.
What triggers auxiliary heat?
The clearest answers come from a September 2025 conference paper by Sugirdhalakshmi Ramaraj and Jon Winkler at NREL, who instrumented ducted heat pumps in cold-climate homes and took airside and power readings every five seconds. They found wide variation. In the 2021-2022 winter, auxiliary heat exceeded 35% of the energy delivered by the compressor at five sites, and at two of them backup heat surpassed compressor heating entirely. At those two homes, the overall seasonal efficiency of the system fell by more than 30%. Six other sites lost less than 7%.
The difference between those groups wasn't the weather. It was mostly setup and controls:
| Cause | What the NREL field data showed |
|---|---|
| Lockout setting ignored | At one site the backup heater engaged at 45°F despite a 35°F lockout setting |
| Undersized heat pump | The same undersized system needed auxiliary heat for roughly 20% to 27% of its heating load |
| Defrost cycles | Backup heat ran during and after defrost at every site, using more than 20% of compressor heating energy at four sites |
| Thermostat setback recovery | Auxiliary heat kicked on while recovering from a setback even with outdoor temperatures above 40°F |
| Undetected faults | A controls fault left one home relying exclusively on backup heat for months |
That last row is the nightmare scenario, and it is more plausible than it sounds. A house heated entirely by strips stays warm. Nothing feels broken. The only symptom is the bill.
Should you turn a heat pump down at night?
This is where advice written for gas furnaces backfires. With a furnace, a deep nighttime setback saves fuel. With a heat pump, the morning recovery can call for a fast temperature rise that the thermostat answers with strips. The Energy Department's instruction is blunt: "Do not set back the heat pump's thermostat if it causes the backup heating to come on." The practical reading is to keep any setback small enough that the morning recovery never calls for strips. If the display shows "aux" every morning, the setback is costing money, not saving it.
Thermostat arguments are hard enough in an office where no setting pleases everyone. At home, the efficient setting with a heat pump is often the boring one: pick a temperature and leave it.
How do I know if my heat strips are stuck on?
Miswiring can make strips run when they shouldn't, even in summer. Building scientist Allison Bailes, writing for GreenBuildingAdvisor, described a 1,700-square-foot house with summer electric bills above $600 because the strip heat ran alongside the air conditioner. His summary of the homeowner's situation: "Your heating and cooling system was doing both at the same time, making you spend a lot of money to stay uncomfortable." The cause, he wrote, "often results from a thermostat wired incorrectly so that the system kicks on the strip heat when it shouldn't."
The utility investigator in that case used a crude but effective test: turn off the breaker for the strip heat and watch how fast the meter moves. Homeowners can do a gentler version by checking whether the thermostat reports auxiliary heat on mild days, and by comparing usage on days of similar weather.
Routine upkeep matters too. The Energy Department says the gap in energy use between a well-maintained heat pump and a severely neglected one "can range from 10% to 25%," and recommends changing filters every three months and professional service at least once a year, including a check that the thermostat is operating properly.
None of this makes heat pumps a bad bet. The physics still favors them by a wide margin, and part of a high bill this winter will have nothing to do with the equipment: rates themselves have been climbing for reasons that include data-center demand, along with line items like the capacity charge. But if a heat pump bill looks wildly out of line with the neighbors', the likeliest culprit is the cheap coil of wire sitting next to it. It was meant to be the understudy. In too many homes, it has quietly taken over the lead.
