One therm of natural gas is equal to 1.1 gallons of propane. For example, at $1.20 per gallon, propane costs $1.32 per therm, compared with 80 cents per therm for natural gas.
One therm of natural gas is equal to 0.7 gallons of heating oil. For example, at $1.00 per gallon, heating oil costs 70 cents per therm, compared with 80 cents per therm for natural gas. But this isn’t quite the whole story. Most heating oil furnaces and stoves are older, with efficiency of about 60%. The newer gas furnaces and boilers use high technology to get efficiencies of 90% and more. Switching to a new gas furnace won’t change the fuel unit cost as much as it will lower the amount of fuel used. Further, about one-third of a residential fuel load is used by the water heater. For those homes with oil heating and electric water heaters, the savings will be even greater (see below).
One therm of natural gas is equal to 29.3 kilowatt-hours of electricity. For example, at 8 cents per kWhr, electricity costs $2.34 per therm, compared with 80 cents per therm for natural gas. Electrical heaters are usually more than 95% efficient. The oldest, least efficient gas appliance will cost less than half as much to run as electric heat. The best heat pumps can’t beat the worst gas appliances.