A year ago, I moved from a condo with a natural gas supply to a home with no natural gas supply. We installed a propane tank for our kitchen range and recently hooked-up a tank water heater to the propane supply. Now, I'm curious to see how natural gas and propane compare on a cost per energy output basis.
Natural gas is often quoted in therms, which is an energy output value corresponding to 100,000 British Thermal Units. A cubic foot of natural gas yields approximately 1,028 BTUs meaning that one therm corresponds to 97.3 cubic feet of natural gas. Propane is stored as a compressed liquid and is typically measured in gallons. Each gallon of propane yields 91,690 BTUs. So, 1.09 gallons of propane provide equivalent energy output to 1 term of natural gas. Then, prices of propane and natural gas are at parity if 1 therm of natural gas costs 9% more than one gallon of propane.
FWIW: My propane delivery company is EnergyUSA Propane.
Update (10/15/10): For reference, a gallon of Diesel provides 128,700 BTUs, appx. 31.7% more than propane per volume. Note that #2 fuel oil (the oil typically used for home heating in the U.S.) is equivalent to Diesel (except for a colorant).
Update (10/21/10): After checking my utility bills, it looks like we paid appx. $1 per therm for natural gas while living in the condo. At the house, we pay appx. $3 per gallon for propane. So, natural gas is easily the cheaper option, costing appx. 30% of the propane price to provide an equivalent amount of BTUs (!) Too bad natural gas isn't an option for us...