In extremely cold weather conditions where the inside thermostat is set to 27 C and the outside temperature is -20 C, your 1200 square ft house might lose heat through the walls at a rate of 3950 W. How much energy might you lose in one week? Electric heat can be 100% efficient. At 10 cents per kilowatt-hour how much would it cost to heat your house for one week under such cold conditions? For an air conditioner used as a heat pump, the coefficient of performance can be as high as the inverse of the Carnot efficiency which is run between the temperature inside (27 C) and the temperature outside (-20 C). How much would it cost to heat your house for one week using such a heat pump? (note that a real air conditionier usually has a COP which is half that for a Carnot cycle) An even more efficient system is a geothermal air conditioner, where the heat pump runs between the inside temperature (27 C) and pipes which are buried underground below the frost line where the temperature is always 59 F or 15 C. How much would it cost to heat your house for one weak using such a geothermal system?