The average portable propane patio heater houses a 20lb propane tank or about 4.7 gallons of fuel. A 20lb propane tank can last up to 10 hours and costs between $15 - $20 to fill up, making the average cost per hour between $1.5 - $2. A typical 40,000 BTU patio heater will cost about $1.70 per hour to use.