house is heated in winter by a biogas boiler. The boiler is rated at 70,000 Btu/h. The heating value of biogas is 13,000 Btu/h, and the average boiler efficiency is 87 percent. Taking the total winter period to be 3000 h and assuming that the boiler operates at an average load of 75 percent, determine the amount of biogas consumed in therm (1 therm = 100,000 Btu) in the winter season. If the biogas is sold at a unit price of $1.20/therm, determine the cost of this biogas to the homeowner.