Estimate the monthly cost of electric auxiliary heat strips and compare it with heat pump heat for the same warmth.
Table of contents
How to use our Heat Pump Auxiliary Heat Cost Calculator
- Enter Auxiliary heat size in kW, using the total heat strip size if your system has more than one strip.
- Enter Electricity price as your all-in $ per kWh if your bill shows energy and delivery charges separately.
- Enter Aux heat runs and Days with aux heat in the month from your thermostat history, energy monitor, or a careful estimate.
- Enter Heat pump COP for comparison; use a lower COP for very cold weather and a higher COP for milder weather if you know your system data.
- Click Calculate, then sanity-check Auxiliary heat electricity used: kW times total hours should match the kWh shown.
Definitions
Auxiliary heat size: The rated electric heat strip power in kW. A larger kW number uses more electricity each hour it runs.
Electricity price: The cost for each kilowatt-hour of electricity, shown as $ per kWh on many electric bills.
Aux heat runs: The average number of hours per day that AUX, auxiliary heat, emergency heat, or heat strips run.
Days with aux heat in the month: The count of days in the month when auxiliary heat ran at least some of the time.
Heat pump COP for comparison: COP means coefficient of performance. A COP of 2.5 means the heat pump delivers about 2.5 kWh of heat for each 1 kWh of electricity it uses.
Auxiliary heat electricity used: The estimated kWh used by the electric heat strips during the runtime you entered.
Extra cost compared with heat pump heat: The estimated added cost of using electric strip heat instead of the heat pump delivering the same heat at the COP you entered.
Common mistakes and quick fixes
Mistake: Entering the outdoor unit size for Auxiliary heat size instead of the electric heat strip size.
Fix: Use the kW rating from the air handler label, service paperwork, or installer notes.
Mistake: Using only the supply energy charge for Electricity price when delivery charges are also billed per kWh.
Fix: Use the all-in $ per kWh if you want a closer bill estimate.
Mistake: Putting total monthly hours into Aux heat runs , which expects hours per day.
Fix: Divide total monthly aux hours by Days with aux heat in the month , then enter that daily average.
Mistake: Entering 30 for Days with aux heat in the month when AUX only appeared during a short cold snap.
Fix: Count only the days when auxiliary heat actually ran.
Mistake: Setting Heat pump COP for comparison too high for very cold weather.
Fix: Use a cold-weather COP estimate if you have manufacturer data, or try a lower COP to see how the comparison changes.
Mistake: Reading Extra cost compared with heat pump heat as your full heating bill increase.
Fix: Treat it as the extra cost for the same heat, compared with the COP you entered.
Limitations & Key Assumptions / Boundary Conditions
- The comparison assumes electric resistance auxiliary heat delivers about the same heat as its electric energy input, so it uses COP 1 for the strip heat comparison.
- The heat pump comparison uses one entered Heat pump COP for comparison. Real COP changes with outdoor temperature, defrost cycles, airflow, and equipment condition.
- The calculator estimates only electric auxiliary heat strip cost. It does not estimate compressor runtime, fan power, gas backup heat, demand charges, taxes, or fixed utility fees.
- If Aux heat runs or Days with aux heat in the month is an estimate, the cost result is only as accurate as that runtime estimate.
- A positive Extra cost compared with heat pump heat does not prove the heat pump could have handled the heating load without auxiliary heat.
- The valid COP range is 1 to 6 for this simple tool. Values outside that range need a more detailed system model.
Methodology
Energy used by the heat strips
The calculator treats the electric heat strip rating as steady electric power while auxiliary heat is running. Electric bills commonly use kilowatt-hours, where 1 kW running for 1 hour uses 1 kWh [1].
aux_energy_used = strip_size_kw * aux_hours_per_day * aux_days_per_month
Auxiliary heat cost
The monthly auxiliary heat cost is the estimated heat strip electricity use multiplied by the entered electricity price.
monthly_aux_heat_cost = aux_energy_used * electricity_rate
The cost for one hour of auxiliary heat is the strip size multiplied by the electricity price.
cost_per_aux_hour = strip_size_kw * electricity_rate
Heat pump comparison
For the comparison, the same delivered heat is assigned to the heat pump. The heat pump electricity needed is the auxiliary heat kWh divided by the entered COP.
same_heat_with_heat_pump_cost = (aux_energy_used / heat_pump_cop) * electricity_rate
The extra cost is the auxiliary heat cost minus the heat pump cost for that same heat.
extra_cost_vs_heat_pump = monthly_aux_heat_cost - same_heat_with_heat_pump_cost
Mini-example
For a 10 kW auxiliary heat strip, $0.17 per kWh electricity, 2 hours per day, 10 days, and heat pump COP 2.5, the strip energy is 200 kWh. Auxiliary heat costs $34.00. The same heat from the heat pump would cost $13.60. The extra cost is $20.40. Each hour of AUX costs $1.70.
Calculation choices
Blank or malformed required inputs are treated as errors. The calculator allows zero runtime, which produces $0 cost. It rejects nonpositive strip size, negative electricity price, hours above 24 per day, days above 31 per month, and COP values below 1 or above 6.