4. A friend offers to sell you an experimental Stirling engine to power your house. The engine is guaranteed to last 15 years and is fueled by burning natural gas with a combined combustion and heat transfer efficiency of 92.0% and a generation efficiency of 97.0%. Your friend tells you that the Stirling engine has an efficiency of 55.0%. Operations and maintenance costs total $0.01/kWh. Assume natural gas costs will be $7.50/MBtu for the next 15 years and an interest rate of 6% and financing for 15 years. Assume your house requires 22 kWh per day and you currently pay $0.18/kWh and you anticipate this will remain constant for the next 15 years. 4b. What is the cost of fuel per kWh