Determine the SPP of installing four smart thermostats in a house in Chicago, IL, at a total cost of $950. The house has an overall BLC of 700 Btu/°F·hr. The house is heated by a furnace with an efficiency of 85 percent and cooled with an AC with COP = 4.0. Assume the electricity price is $0.12/kWh, the natural gas price is $1.75/therm, and that the smart thermostats reduce home thermal loads by 22 percent for heating and by 18 percent for cooling.