Heat Pump Running Cost Calculator

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Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Heat moved during the season: The amount of heat the system adds to the home in heating mode or removes from the home in cooling mode.

Btu: British thermal unit, a common unit for heat energy.

Therm: A gas-bill heat unit equal to 100,000 Btu.

kWh of heat: Heat energy measured in kilowatt-hours. This is delivered or removed heat, not the heat pump's electric use.

COP: Coefficient of performance. A COP of 3 means the heat pump moves about 3 units of heat for each 1 unit of electricity.

HSPF2: Heating Seasonal Performance Factor 2, a heating efficiency rating in Btu per watt-hour. It is one of the heating ratings shown on heat pump efficiency labels [1].

SEER2: Seasonal Energy Efficiency Ratio 2, a cooling efficiency rating in Btu per watt-hour.

Electric backup heat share: The percent of heating supplied by electric resistance backup heat, modeled at COP 1.

Cost per million Btu of heat moved: The cost to move the same amount of heat, scaled to 1,000,000 Btu so fuels and ratings are easier to compare.

Break-even COP for current heat: The COP where heat pump electricity cost would equal the current heater cost for the same delivered heat.

Mistake: Putting electric kWh used into Heat moved during the season.
Fix: Use Heat moved unit to enter Btu, therms, or kWh of heat delivered or removed, not the electricity used by the heat pump.

Mistake: Choosing SEER2 but leaving Heat pump rating as a COP value like 3.0.
Fix: Match Heat pump rating to What are you estimating?; SEER2 and HSPF2 are usually larger Btu per watt-hour ratings.

Mistake: Entering a bare whole number such as 18, which could mean dollars or cents.
Fix: Enter the usual dollar form as 0.18 or $0.18. Explicit cents forms such as 18¢ and 18 cents are also accepted.

Mistake: Adding resistance backup usage to an HSPF2 estimate as if HSPF2 described only the compressor. Fix: Use "Heating with HSPF2" for a standardized whole-season estimate. Use "Heating with COP" with "Electric backup heat share" for a separate component scenario.

Mistake: Reading Savings compared with current heat as always positive.
Fix: Positive Savings compared with current heat means the heat pump is cheaper; a negative value means the heat pump costs more for the same heat.

Mistake: Entering the wrong Current fuel price unit for Compare with current heat.
Fix: Use $ per therm for natural gas, $ per gallon for propane or heating oil, and $ per kWh for electric resistance heat.

• The result depends on Heat moved during the season. If that number comes from an old fuel bill, adjust for non-heating uses such as gas water heating or cooking before using it.
• Electric rate should include delivery and supply charges when possible. Fixed monthly fees are not included.
• Season length only spreads Estimated season cost into Average cost per month in season; it does not change the total energy use.
• "Electric backup heat share" applies only in "Heating with COP". "Heating with HSPF2" and both cooling modes hide and ignore it. An HSPF2 label is a standardized seasonal comparison, not a forecast of the actual backup usage in your home.
• The current-fuel comparison uses approximate fuel heat content and Current heater efficiency. Real equipment can vary because of cycling, ducts, controls, and maintenance.
• Current heater cost for same heat is operating cost only. It excludes equipment price, repairs, rebates, service charges, and fuel tank or meter fees.
• Break-even COP for current heat ignores backup heat so it can be compared with a heat pump's operating COP.

How the calculator works

The calculator first converts Heat moved during the season into Btu. It uses 1 Btu per Btu, 100,000 Btu per therm, and 3,412.141633 Btu per kWh of heat.

heat_btu = load_amount * unit_factor

For COP modes, electricity use is heat moved divided by COP and the Btu per kWh conversion.

heat_pump_kwh = heat_btu / (cop * 3412.141633)

For HSPF2 or SEER2 modes, the rating is Btu per watt-hour, so the formula multiplies by 1000 to convert watt-hours to kWh.

heat_pump_kwh = heat_btu / (rating_btu_per_wh * 1000)

In "Heating with COP", "Electric backup heat share" splits delivered heat between the heat pump and resistance heat at COP 1. "Heating with HSPF2" instead divides the whole seasonal load by HSPF2 × 1,000, without adding backup heat again.

total_kwh = (heat_btu * (1 - backup_share) / useful_rating_divisor) + (heat_btu * backup_share / 3412.141633)

The season cost uses the entered Electric rate in dollars per kWh.

season_cost = total_kwh * electricity_rate_dollars

The average monthly result and cost-per-heat result are simple follow-on calculations.

average_monthly_cost = season_cost / season_months

cost_per_million_btu = season_cost / (heat_btu / 1000000)

Effective COP used is the delivered or removed heat divided by the electric energy used, after any backup heat share is included.

effective_cop = heat_btu / (total_kwh * 3412.141633)

Fuel comparison math

When Compare with current heat is selected, the calculator prices the same delivered heat with the selected fuel and Current heater efficiency.

compare_cost = heat_btu / (fuel_btu_per_unit * efficiency) * fuel_price_per_unit

It keeps Savings compared with current heat signed, because a negative value is useful information.

savings_vs_current_heat = compare_cost - season_cost

The break-even COP shows the heat pump COP that would tie the current heater cost under the entered prices.

break_even_cop = electric_price_per_kwh * fuel_btu_per_unit * efficiency / (fuel_price_per_unit * 3412.141633)

Mini example

If heating load is 36,000,000 Btu, COP is 3.0, Electric rate is $0.18 per kWh, and Season length is 5 months, estimated electricity is 36,000,000 / (3.0 * 3,412.141633) = about 3,516.85 kWh. The Estimated season cost is 3,516.85 * 0.18 = about $633.03, and the Average cost per month in season is about $126.61.