Heat Pump vs Boiler Calculator

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Table of contents

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

MMBtu: One million British thermal units, a common way to measure heat energy.

COP: Coefficient of performance. For heating, it is the heat output divided by electric power input; a seasonal COP of 3 means about 3 units of heat for 1 unit of electricity [1].

Boiler efficiency: The percent of fuel heat that becomes useful heat in the home. An 85 percent boiler delivers 85 percent of the fuel heat.

Heat pump share of heating: The percent of yearly space-heating heat supplied by the heat pump. The rest is treated as boiler heat.

Fixed charge: A monthly fuel account charge paid even before using fuel. It is separate from the fuel price per therm, gallon, or kWh.

Break-even COP: The seasonal COP where heat pump electricity cost equals boiler fuel cost before fixed charges.

Break-even electricity price: The $ per kWh price where the heat pump and boiler have the same heating-only cost at the COP entered.

Mistake: Entering total home energy use in Heat needed for space heating, including water heating or cooking.
Fix: Use space-heating heat only, or expect the Boiler heating cost and Heat pump setup heating cost to be too high.

Mistake: Choosing Natural gas in Boiler fuel type but entering a Boiler fuel price from an oil or propane bill.
Fix: Match Boiler fuel type to the unit on the bill, then enter the price per selected unit.

Mistake: Typing Boiler efficiency as 0.85 instead of 85.
Fix: Enter Boiler efficiency as a percent from 0 to 100, such as 85 for an 85 percent boiler.

Mistake: Using an Electricity price that includes fixed customer charges in $ per kWh.
Fix: Use the kWh-based electricity price for Electricity price, and keep fixed fuel account charges in Current boiler fuel fixed charge and Fixed charge after switching.

Mistake: Setting Heat pump share of heating to 100 when the boiler will still run on cold days.
Fix: Enter the expected Heat pump share of heating, such as 80, so the remaining boiler fuel cost stays in the comparison.

Mistake: Reading a negative Annual savings after fixed charges as an error.
Fix: A negative Annual savings after fixed charges means the heat pump setup costs more per year with the prices and charges entered.

• The calculator compares space-heating operating cost only. It does not include equipment purchase price, rebates, financing, maintenance, repairs, or cooling value.
• Heat needed for space heating must be a yearly useful heat load. If the input includes water heating, cooking, or pool heating, the comparison will be too high.
• Seasonal COP is treated as one average number for the year. Real heat pump COP changes with outdoor temperature, indoor temperature, defrost cycles, and equipment sizing.
• Boiler efficiency is treated as one average useful-heat percentage. Real performance can change with return-water temperature, cycling, maintenance, and distribution losses.
• Fuel heat content uses fixed values for natural gas therms, heating oil gallons, propane gallons, and electric kWh. Actual delivered fuel heat can vary by supplier and fuel mix.
• Fixed monthly charges are modeled only for the boiler fuel account. Other electric bill customer charges, demand charges, taxes, and rate tiers are not separately modeled.
• Break-even COP and break-even electricity price ignore fixed monthly charges. Use Annual savings after fixed charges for the main yearly bill comparison.

How the yearly costs are calculated

The calculator first converts the selected boiler fuel into heat units. It uses 0.1 MMBtu per natural gas therm, 0.1385 MMBtu per heating oil gallon, 0.091452 MMBtu per propane gallon, and 0.003412141633 MMBtu per kWh. Electricity prices can include several bill parts, so the Electricity price input should be the part that changes with kWh use [2].

boiler_fuel_use_year = annual_heat_mmbtu / ((boiler_efficiency_percent / 100) * fuel_mmbtu_per_unit)

boiler_heating_cost_year = boiler_fuel_use_year * boiler_fuel_price

Heat pump electricity use depends on the yearly heat served by the heat pump and the seasonal COP entered. The remaining heat share, if any, stays with the boiler.

heat_pump_share = heat_pump_heat_share_percent / 100

heat_pump_electricity_kwh_year = (annual_heat_mmbtu * heat_pump_share) / (heat_pump_cop * kwh_to_mmbtu)

heat_pump_setup_cost_year = heat_pump_electricity_kwh_year * electricity_price_kwh + boiler_heating_cost_year * (1 - heat_pump_share)

Savings are signed. Positive savings mean the heat pump setup costs less. Negative savings mean it costs more.

annual_savings_before_fixed = boiler_heating_cost_year - heat_pump_setup_cost_year

annual_fixed_charge_savings = 12 * (current_fixed_charge_monthly - after_switch_fixed_charge_monthly)

annual_savings_after_fixed = annual_savings_before_fixed + annual_fixed_charge_savings

Break-even values

The break-even outputs compare energy-use cost before fixed monthly fuel charges. Break-even heat pump COP is the seasonal COP needed for the heat pump electricity cost to match the boiler fuel cost. Break-even electricity price is the $ per kWh price that would make the two heating-only costs equal at the COP entered.

break_even_cop_before_fixed = electricity_price_kwh / ((boiler_fuel_price / ((boiler_efficiency_percent / 100) * fuel_mmbtu_per_unit)) * kwh_to_mmbtu)

break_even_electricity_price = heat_pump_cop * kwh_to_mmbtu * (boiler_fuel_price / ((boiler_efficiency_percent / 100) * fuel_mmbtu_per_unit))

Mini-example

For 60 MMBtu/year, natural gas at $1.50 per therm, an 85 percent boiler, electricity at $0.18 per kWh, COP 3.0, and a 100 percent heat pump share, boiler fuel use is about 705.88 therms/year and boiler heating cost is about $1,058.82/year. Heat pump electricity use is about 5,861.42 kWh/year and heat pump setup heating cost is about $1,055.06/year. With no fixed charge change, Annual savings after fixed charges is about $3.77/year.