Boiler Running Cost Calculator

Related Calculators

Table of contents

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

Boiler fuel: The energy source your boiler uses. This calculator supports natural gas, propane, heating oil, and electricity.

Heat delivered to the home: Useful heat sent into the house, measured in BTU per hour. BTU means British thermal unit, a common heat-energy unit.

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

Fuel price: Your local price for the selected fuel unit: $ per therm, $ per gallon, or $ per kWh.

Therm: A natural gas energy unit equal to 100,000 BTU.

Fuel used per running hour: The amount of selected fuel the boiler needs for one active running hour after efficiency is included.

Fuel energy burned per running hour: The fuel heat going into the boiler before efficiency losses.

Mistake: Entering Fuel price in the wrong unit, such as $ per cubic foot for natural gas instead of $ per therm.
Fix: Use $ per therm for natural gas, $ per gallon for propane or heating oil, and $ per kWh for electricity.

Mistake: Using burner input for Heat delivered to the home when the label asks for useful heat sent into the house.
Fix: Use the boiler output rating if you have it, or adjust the input rating by the Boiler efficiency before entering it.

Mistake: Leaving Boiler efficiency blank or typing a value above 100 percent.
Fix: Enter a percent from just above 0 through 100, such as 85 for an 85 percent efficient boiler.

Mistake: Treating Boiler run time as the full 24 hours the thermostat is on.
Fix: Enter only the hours the burner or electric heating element is actually running during a heating day.

Mistake: Keeping Heating days in a month at 30 for a mild month with only a few heating days.
Fix: Change Heating days in a month in Advanced options so Estimated boiler fuel cost per heating month matches the period you mean.

Mistake: Reading Estimated boiler fuel cost for heating season as a weather forecast.
Fix: Treat it as a schedule-based estimate from Heating months per year, not a prediction of future outdoor temperatures.

• The estimate uses the run time you enter. It does not model thermostat cycling, outdoor temperature changes, wind, sunlight, insulation, or setback schedules.
• Heat delivered to the home must be useful output heat. If you enter boiler input heat instead, the calculator will count efficiency losses twice.
• Fuel price must match the selected Boiler fuel unit. Utility bills may also include fixed customer charges, taxes, delivery fees, minimum charges, or tiered rates that are not included.
• Fuel heat contents are standard reference values. Actual fuel energy can vary by supplier, region, and fuel grade.
• Heating days in a month and Heating months per year are schedule inputs, not climate-normalized averages.
• The calculator estimates fuel cost only. It does not include maintenance, repair, electricity for pumps or controls on fuel-fired boilers, or boiler standby losses beyond the entered efficiency.

Calculation steps

The calculator first changes Boiler efficiency from a percent to a decimal. It then finds the fuel heat needed to deliver the entered heat output, converts that fuel heat to therms, gallons, or kWh, and multiplies by Fuel price. The fuel heat constants used are 100,000 BTU per therm for natural gas, 91,452 BTU per gallon for propane, 138,500 BTU per gallon for heating oil, and 3,412.141633 BTU per kWh for electricity [1][2].

efficiency_decimal = boiler_efficiency_percent / 100

fuel_energy_btu_per_hour = heat_delivered_btu_per_hour / efficiency_decimal

fuel_used_per_running_hour = fuel_energy_btu_per_hour / btu_per_fuel_unit

cost_per_running_hour = fuel_used_per_running_hour * fuel_price

cost_per_heating_day = cost_per_running_hour * run_hours_per_day

cost_per_heating_month = cost_per_heating_day * heating_days_per_month

cost_per_heating_season = cost_per_heating_month * heating_months_per_year

Mini-example

For a natural gas boiler delivering 60,000 BTU per hour at 85 percent efficiency, the fuel energy needed is 60,000 / 0.85 = 70,588.235 BTU per running hour. Natural gas is counted at 100,000 BTU per therm, so fuel use is 70,588.235 / 100,000 = 0.7059 therms per hour. At $1.50 per therm, the running cost is 0.7059 * 1.50 = $1.06 per hour. If the boiler runs 8 hours per heating day, the heating-day cost is $8.47. With 30 heating days in a month and 5 heating months per year, the month estimate is $254.12 and the season estimate is $1,270.59.

Rounding and validation

Money outputs are rounded for display, but the period totals are calculated from the unrounded running-hour cost. The calculator blocks blank required inputs, negative Fuel price, Boiler efficiency at or below 0 or above 100, Boiler run time above 24 hours, Heating days in a month above 31, and Heating months per year above 12.