After your initial purchase, the cost of operation depends on your fuel type. Propane is generally the most expensive, natural gas is typically the least expensive, and electricity is in between.Natural Gas: A typical 40,000 BTU patio heater that heats an area of 314 sq ft will cost an average of 48 cents per hour to use.Electric: The average cost for a 4,000-watt electric patio heater is 60 cents per hour (15 cents per kilowatt hour).Propane: A typical 40,000 BTU patio heater will cost about $1.70 per hour to use.Gas and electricity rates may vary depending on where you live and the efficiency of your patio heaters.