If your heat pump uses electric resistance strips for backup, estimate their electricity cost from the kilowatts actually running and the hours they ran. The AUX indicator is a clue, not a meter. It does not tell you which stages ran, and some systems use a gas furnace instead of electric strips.
Identify the backup source
Look up the equipment model or ask your HVAC contractor which backup heat is installed. Do not open an electrical compartment to check. Ask whether a thermostat’s runtime report distinguishes auxiliary stages from compressor operation. If the answer is no, treat an estimate using the maximum installed strip rating as a scenario rather than as a bill reconstruction.
Compare two runtime scenarios
| Backup operation | Estimated electricity | Cost at $0.18/kWh |
|---|---|---|
| 5 kW for 2 hours | 10 kWh | $1.80 |
| 10 kW for 2 hours | 20 kWh | $3.60 |
| 5 kW for 2 hours on 10 days | 100 kWh | $18.00 |
The table assumes full power for the stated hours. For a thermostat that logs different stages, total the energy from each interval using the combined power active in that interval. If the compressor used 8 kWh on the same day as the first example, the two loads together used 18 kWh, but only 10 kWh was attributed to the strips. Use the variable price per kWh, including applicable per-kWh charges; a fixed monthly customer charge does not change when strips run.
What to ask before paying for service
Record outdoor temperature, thermostat mode, setpoint changes, and the dates and hours shown for backup. Note any unusually cold mornings or loss of comfort. Ask the contractor to verify staging and controls against the specific equipment rather than turning backup off. The calculation helps price runtime; it does not diagnose a fault or estimate the exact savings from changing thermostat settings.
Related: Heat Pump Thermostat Setback: Does Morning Recovery Erase the Savings?.
Source: U.S. Department of Energy: Heat Pump Systems. Numerical examples are hypothetical.