"If one gallon of propane is equal to 27 kWh of electricity, then we can compare the costs of these fuels directly by looking at the price per unit (propane gallons or kilowatt hours) and finding the price difference. This can easily be done by looking at your electric bill and multiplying the price per kWh by 27. The resulting number will be a dollar figure that will be either greater than or less than the price of a gallon of propane. For example, if you are paying 12ยข per kWh, the electrical cost comparison figure to a gallon of propane will be $3.24 (.12 x 27 = 3.24). Electricity is cheaper than propane if propane is selling for $3.24 per gallon or more and propane is cheaper than electricity if it is selling for less than $3.24 per gallon."
Of course a special trip to get propane tank refilled needs to be account for. I try to include that in other shopping trips.
Chris