A residential heat pump can draw from a few hundred watts at low speed to several kilowatts at full output. Electric backup heat can add 5 to 20 kilowatts or more.
The useful number is not a generic wattage. It is the input power of the exact indoor and outdoor equipment at the temperature and operating stage you care about.
Typical heat-pump power ranges
A small ductless unit may draw roughly 500 to 2,500 watts while running. A central ducted system may draw about 2,000 to 7,000 watts across its operating range. Variable-speed equipment can run well below full input for long periods.
Those are screening ranges, not circuit-sizing values. Capacity, climate, fan power, compressor speed, duct pressure and outdoor temperature all affect demand.
Electric resistance strips are different. A 10-kilowatt heat kit draws about 10,000 watts when fully energised, which can exceed the compressor and fan demand combined.
Calculate watts from heating output and COP
Coefficient of performance, or COP, is useful when heating capacity and efficiency are known:
Electrical input in watts = heating output in Btu per hour ÷ 3,412 ÷ COP.
A heat pump delivering 36,000 Btu per hour at a COP of 3 would use about:
36,000 ÷ 3,412 ÷ 3 = 3,517 watts.
This is a steady-condition estimate. Defrost, crankcase heat, fans, pumps and backup heat can change total input.
Calculate watts from electrical data
For a single-phase AC load:
Real power = volts × amps × power factor.
Simply multiplying volts by amps gives volt-amperes, not always true watts. Motors and inverter-driven compressors can have a power factor below 1, and current changes with operating speed.
Use measured data from qualified equipment or manufacturer performance tables. The minimum circuit ampacity and maximum overcurrent protection on the nameplate are for wiring and breaker selection, not estimates of normal energy use.
Why the same heat pump uses different power
- Outdoor temperature: Heating capacity and COP change as the air gets colder.
- Compressor speed: Variable-speed systems modulate instead of drawing one fixed amount.
- Auxiliary heat: Resistance strips may stage on during cold weather, recovery or defrost.
- Airflow: Dirty filters, restrictive ducts and incorrect fan settings increase system work.
- Defrost: The unit periodically reverses operation to clear ice from the outdoor coil.
- Ground-loop pumps: Geothermal systems include circulating-pump demand.
Estimate daily and monthly electricity use
Energy in kilowatt-hours equals average power in kilowatts multiplied by operating hours.
If a heat pump averages 2.4 kilowatts for 10 hours, it uses 24 kilowatt-hours. At an electricity price of $0.16 per kilowatt-hour, that day costs $3.84.
Do not multiply maximum nameplate input by thermostat hours unless the unit actually runs continuously at maximum output. A whole-home energy monitor, utility interval data or a technician’s measurement gives a better estimate.
What to use for generator or panel sizing
Use the equipment nameplate, approved combination data and installation instructions. Include the outdoor unit, indoor fan, pumps, crankcase heater and any staged resistance heat. Compressor starting demand also matters unless the system uses a compatible inverter or soft-start arrangement.
Have an electrician or HVAC designer calculate the load. Do not size a generator from average monthly energy consumption.
Bottom line: most of the uncertainty comes from modulation and backup heat. Use model-specific performance data for operating cost and nameplate requirements for electrical design.
Sources: U.S. Department of Energy heat-pump overview, DOE efficiency definitions and ENERGY STAR air-source heat pumps.