How to compare boiler fuel costs
Natural gas is billed by the therm, oil and propane by the gallon, and electricity by the kilowatt-hour, so the posted prices cannot be compared directly. The only meaningful yardstick is what it costs to deliver the same amount of heat into the building. This calculator converts every fuel to dollars per MMBtu, then shows monthly usage and monthly cost side by side.
The heating values applied are 100,000 Btu per therm of natural gas, 138,500 Btu per gallon of No. 2 fuel oil, 91,500 Btu per gallon of propane, and 3,412 Btu per kilowatt-hour of electricity. Each is multiplied by the efficiency you enter to get delivered heat, and the unit price is divided by that figure. At $1.20 per therm and 85 percent efficiency, natural gas delivers heat at about $14.12 per MMBtu, while resistance electricity at $0.15 per kWh lands near $43.96.
Real bills also carry customer charges, tiered or seasonal rates, delivery fees, and taxes, and utility heat content varies slightly by region. Switching fuels means paying for new equipment, venting, and possibly a storage tank, so compare total cost of ownership rather than heat cost alone, and have a mechanical contractor size and quote the replacement before committing.
Frequently asked questions
No. Each fuel packs a different amount of heat into its billing unit, and boiler efficiency differs too. The only fair comparison is the delivered cost of the same amount of heat, which is what the dollars per MMBtu column shows.
Condensing gas boilers commonly rate around 90 to 95 percent AFUE, while older atmospheric gas and oil units fall near 80 to 85 percent. Use the AFUE from the unit nameplate or the ENERGY STAR listing rather than a catalog peak figure.