Every cold snap produces the same wave of complaints: heat pump owners whose bills doubled without them touching the thermostat. It isn't a billing error, and the heat pump isn't broken in most cases.
It's the resistance strips sitting behind it, which switch on automatically and cost roughly three and a half times as much per hour to run.
The Mechanism
A heat pump doesn't generate heat, it moves it — delivering about 2.5 to 3.5 units of heat per unit of electricity. That's why it's cheap to run in mild weather.
But its output falls as the outdoor temperature drops. At some point — the balance point — it can no longer keep pace with what the house is losing. Below that, the thermostat brings in auxiliary heat: electric resistance strips in the air handler, the same technology as a hair dryer, delivering exactly one unit of heat per unit of electricity.
The cost difference at the 2026 U.S. average of 18.4¢/kWh, with a 4 kW heat pump and a typical 10 kW strip bank:
- Heat pump alone: 4 kW → about 74¢/hour
- Heat pump + auxiliary: 14 kW → about $2.58/hour
A hundred hours of auxiliary runtime in a cold month — routine during a prolonged freeze — adds roughly $184 to the bill. That's the whole explanation. Your usage genuinely rose; the meter isn't lying.
You can work out your own figure with our auxiliary heat cost calculator, which also flags whether AUX is running more than it should for your climate.
What's Normal and What Isn't
Some auxiliary runtime is expected and correct:
- Standard heat pump: balance point typically 30–40°F
- Cold-climate / variable speed: often 5–15°F
- Defrost cycles: brief AUX runs while the outdoor coil de-ices — normal and short
What isn't normal is the AUX indicator staying lit in mild weather, or engaging above roughly 45°F. Common causes, roughly in order of frequency:
- Thermostat lockout never configured. Most thermostats can be set so strips are blocked above a given outdoor temperature. If nobody set it, the strips are free to come on whenever the thermostat feels behind.
- A large thermostat setback — covered below, and the one homeowners cause themselves.
- Low refrigerant charge, which cuts capacity so the heat pump falls behind earlier than designed.
- Failed defrost board or outdoor fan, leaving the coil iced and the heat pump producing little.
- Reversing valve stuck in cooling — the heat pump contributes nothing and strips do all the work.
A burning-dust smell the first time strips run each autumn is normal. Continuous AUX in 50°F weather is a service call.
The Setback Mistake
This one catches people who followed standard advice, and it's worth stating plainly because most thermostat guidance assumes a furnace.
With a gas furnace, a deep overnight setback saves money — it reheats quickly at normal efficiency. With a heat pump, recovery is slow by design. A thermostat that wakes to find the house 8°F below setpoint concludes the heat pump can't cope and calls in the strips to catch up. You then pay resistance rates through the whole recovery, which can exceed what the setback saved overnight.
For heat pumps: keep setbacks to 2–3°F, not the 7–10°F standard advice suggests, and if your thermostat has an adaptive or smart recovery mode built for heat pumps, leave it on rather than running a manual schedule.
Our thermostat settings guide and personal heating savings calculator use the DOE setback rule, which assumes a furnace — if you have a heat pump with strips, treat those numbers as an upper bound.
Never Use Emergency Heat Because It's Cold
Auxiliary and emergency heat use the same strips but behave completely differently, and the labels invite exactly the wrong action.
Auxiliary heat is automatic. The thermostat brings strips in to help the heat pump, which keeps running.
Emergency heat is a manual setting that shuts the heat pump off and runs strips alone. It's for when the heat pump is broken or being serviced.
Seeing "EM HEAT" on the thermostat during a freeze and switching it on feels like the responsible move. It isn't — you're discarding the efficient part of your system exactly when you need it most, and it's usually the single most expensive setting available to you. We cover the distinction in detail in auxiliary heat vs emergency heat.
What Actually Reduces the Bill
- Set the outdoor lockout. Ask your installer to configure a lockout just below your balance point so strips can't be used as a shortcut on mild days. Often the single biggest fix, and it costs nothing.
- Shrink your setbacks, per above.
- Change the filter. Restricted airflow makes the system fall behind and reach for strips. Cheapest item on this list.
- Air sealing and insulation. These lower the balance point itself — a tighter house stays ahead of its losses to a colder outdoor temperature, so strips engage less often. Our insulation savings calculator covers the return.
- Have the charge and defrost checked if AUX runs in mild weather. That's diagnosis, not tuning.
Is the Heat Pump Still Worth It?
Worth saying clearly, because these bills prompt regret: a correctly configured heat pump that rarely touches its strips remains substantially cheaper to run than electric resistance heat, and competitive with gas in most of the country. The problem in nearly every "my bill doubled" case is the strips running more than they should, not the heat pump concept failing.
If you're reassessing the whole decision, our heat pump vs furnace vs boiler comparison puts running costs side by side, and heat pump economics without the tax credit covers where the numbers landed after the 30% federal credit expired at the end of 2025.
And if your bill rose but your usage didn't, the cause may not be your heating at all — rate increases have been steep. Our bill increase breakdown calculator separates a usage change from a rate change.