Why Strips Cost So Much More
A heat pump doesn't make heat, it moves it — delivering roughly 2.5 to 3.5 units of heat for every unit of electricity. Auxiliary strips are electric resistance, the same principle as a hair dryer, delivering exactly one unit per unit. So the strips aren't slightly worse, they're two to three times worse per unit of heat, and they draw far more power to boot.
At the 2026 U.S. average of 18.4¢/kWh, with a 4 kW heat pump and a 10 kW strip bank:
- Heat pump alone: 4 kW → about 74¢/hour
- Heat pump + 10 kW aux: 14 kW → about $2.58/hour (3.5× more)
- Emergency heat, 15 kW strips alone: about $2.76/hour
A hundred hours of aux runtime in a cold month — which is not unusual during a prolonged freeze — adds roughly $184 to the bill. That's the whole explanation for "my heat pump bill doubled and I didn't touch the thermostat."
When AUX Is Normal and When It Isn't
Some auxiliary runtime is expected and correct. A conventional heat pump's output falls as it gets colder, and at some outdoor temperature — the balance point — it can no longer match what the house is losing. Below that, strips fill the gap. Typical balance points:
- Standard heat pump: roughly 30–40°F
- Cold-climate / variable speed: often 5–15°F
Aux also runs briefly during defrost cycles, when the outdoor coil ices up and the system temporarily reverses to melt it. That's normal and short.
What isn't normal is AUX engaging at 45°F or above, or staying lit for hours in mild weather. The usual causes, roughly in order of how often they turn out to be the answer:
- Thermostat lockout set wrong or missing. Many thermostats have an outdoor-temperature lockout that prevents strips running above a set point. If it was never configured, the strips are free to come on whenever the thermostat feels behind.
- A large thermostat setback. Covered below — this is the one homeowners cause themselves.
- Low refrigerant charge, which cuts capacity so the heat pump falls behind earlier than it should.
- A failed defrost board or outdoor fan, leaving the coil iced and the system unable to produce heat.
- A reversing valve stuck in cooling, in which case the heat pump is contributing nothing and strips are doing all the work.
A burning-dust smell when strips first run in autumn is normal. Continuous AUX in mild weather is a service call, not a setting.
The Setback Trap
This is the one that catches people who did everything else right, and it directly contradicts standard thermostat advice.
With a gas furnace, a deep overnight setback saves money — the furnace reheats the house quickly at its normal efficiency. With a heat pump, recovery is slow by design. A thermostat that wakes up and sees the house 8°F below setpoint concludes the heat pump can't cope and calls in the resistance strips to catch up. You then pay strip rates for the entire recovery period, which can exceed everything the setback saved overnight.
Practical guidance for heat pump owners: keep setbacks to 2–3°F rather than the 7–10°F that works for furnaces, and if your thermostat has an adaptive or smart recovery mode built for heat pumps, leave it enabled rather than running a manual schedule.
Our personal heating savings calculator and thermostat settings guide use the standard DOE setback rule, which assumes a furnace. If you have a heat pump with strips, treat those recommendations as an upper bound and scale down.
What Actually Reduces AUX Runtime
- Set the outdoor lockout. Most thermostats let you specify a temperature above which strips cannot run. Setting it just below your balance point stops the strips being used as a shortcut on mild days. Worth asking your installer to configure if you're not sure.
- Shrink or remove setbacks, per above.
- Reduce the heat loss. Air sealing and insulation lower the balance point itself — a tighter house stays ahead of its losses to a colder outdoor temperature, so strips engage less often. See our insulation savings calculator.
- Get the charge and defrost checked if AUX runs in mild weather. This is diagnosis, not efficiency tuning.
- Change the filter. Restricted airflow makes the system fall behind and reach for strips. Cheapest thing on this list.
If you're weighing whether the heat pump was the right call at all, our heat pump vs furnace vs boiler comparison and heat pump economics without the tax credit cover the wider picture — and note that a well-configured heat pump that rarely touches its strips is still cheaper to run than resistance heat by a wide margin.