To find your gas rate, divide your total monthly bill ($) by the total usage (therms).

As a heating source measured in British thermal units (Btu), a gallon of heating oil has 40% more energy than a therm of gas. There are 100,000 Btu in a therm of gas and 140,000 Btu in a gallon of heating oil. You must use 1.4 therms of gas to generate the heat content of a gallon of heating oil. Therefore, to account for the Btu difference between a therm of gas and a gallon of heating oil, you must multiply the price per therm by 1.4. This gives you the real cost of gas heat compared to oil — the most valid cost when comparing the two fuels.

Compare Natural Gas – sold in therms (100,000 BTU/therm)¹
Multiply the oil heat price per gallon by 0.72 to give the equivalent price per therm of natural gas
Multiply the propane price per gallon by 1.087 to give the equivalent price per therm of natural gas
Multiply the electricity price per kWh by 29.3 to give the equivalent price per therm of natural gas

Compare Propane – sold in gallons (92,000 BTU/gallon)
Multiply the oil heat price per gallon by 0.663 to give the equivalent price per gallon of propane
Multiply the natural gas delivered price per therm by 0.92 to give the equivalent price per gallon of propane
Multiply the electricity price per kWh by 27.0 to give the equivalent price per gallon of propane  

Compare Heating Oil – sold in gallons (138,700 BTU/gallon)
Multiply the propane price per gallon by 1.507 to give the equivalent price per gallon of heating oil
Multiply the natural gas delivered price per therm by 1.387 to give the equivalent price per gallon of heating oil
Multiply the electricity price per kWh by 40.6 to give the equivalent price per gallon of heating oil  

Compare Electricity – sold in kilowatt hours (3,413 BTU/kilowatt hour)²
Multiply the propane price per gallon by 0.037 to give the equivalent price per kilowatt hour of electricity
Multiply the natural gas delivered price per therm by 0.034 to give the equivalent price per kilowatt hour of electricity
Multiply the heating oil price per gallon by 0.024 to give the equivalent price per kilowatt hour of electricity  

¹Figure out your price per therm by dividing your total fuel bill by the total therms of natural gas consumed.  This is your price per therm
² Figure out your price per kWh by dividing your total fuel bill by the total kWh of electricity consumed.  This is your price per kWh.

Sources:

Fuel Prices: Energy Information Agency, Dept. of Energy 2005 averages for the North-East for 12 months ended December, 2004 (PADD district 1A includes states CT, ME, MA, NH, RI, VT).

Pollutants: Pollutant level measured in particulates (pounds per million BTU of heat delivered). Data from Dept. of Energy, Energy Information Agency, and Pace University "Environmental Cost of Electricity" and U.S. Environmental Protection Agency. Heat pump and electric heat based on generating plant emissions.

*Other: When electric resistance is used to supplement the heat pumps output in cold weather, heating costs will increase toward "Electric Heat" figures above. Heat Pump heating seasonal performance factor of 1.56 ref. Oak Ridge National Laboratories for U.S. Dept of Energy. More efficient seasonal performance factor of 2.0 used in this analysis.

Data used in table: Fuel Oil ($1.52/gallon with 138,700 BTU/gallon and .0027 lbs./MMBTU particulates); Natural Gas ($1.33/therm with 100,000 BTU/therm and .0030 lbs./MMBTU particulates); Propane ($2.03/gallon with 92,000 BTU/gallon and .0040 lbs./MMBTU particulates); Heat Pump ($0.12/Kilowatt hour with 3,413BTU/kWh and .0145 lbs./MMBTU particulates); Electric Heat ($.12/kWh with 3,413 BTU/kWh and .0290 lbs./MMBTU particulates)

Note: Propane fuel data is based only on winter months October through February of 2005.