A method of operating a vapor compression system that has a compressor and an evaporator, wherein the vapor compression system is configured to produce heat during a heating cycle and defrost the evaporator of the vapor compression system during a defrosting cycle, the method comprising: determining a total heat delivered, THD, by the vapor compression system following a completion of a defrosting cycle; determining a total electrical energy consumed, TEC-H, by the vapor compression system while delivering heat following completion of the defrosting cycle; maintaining a total electrical energy consumed, TEC-D, by the vapor compression system during the defrosting cycle; determining a cumulative coefficient of performance, CCOP, of the vapor compression system based at least in part on the total heat delivered, THD, by the vapor compression system following the completion of the defrosting cycle, the total electrical energy consumed, TEC-H, by the vapor compression system while delivering heat following the completion of the defrosting cycle, and the total electrical energy consumed, TEC-D, by the vapor compression system during the defrosting cycle; and initiating a next defrosting cycle at a time that is based at least in part on one or more characteristics of the cumulative coefficient of performance, CCOP,, wherein the CCOP is determined by dividing the total heat delivered, THD, by the sum of the total electrical energy consumed, TEC-H, by the vapor compression system while delivering heat plus the total electrical energy consumed, TEC-D, by the vapor compression system during the defrosting cycle; and wherein the total electrical energy consumed, TEC-D, by the vapor compression system during the defrosting cycle is an average of the total electrical energy consumed, TEC-D, by the vapor compression system during a previous "N" of the defrosting cycle, wherein "N" is an integer greater than 1. determining a total heat delivered, THD, by the vapor compression system following a completion of a defrosting cycle; determining a total electrical energy consumed, TEC-H, by the vapor compression system while delivering heat following completion of the defrosting cycle; maintaining a total electrical energy consumed, TEC-D, by the vapor compression system during the defrosting cycle; determining a cumulative coefficient of performance, CCOP, of the vapor compression system based at least in part on the total heat delivered, THD, by the vapor compression system following the completion of the defrosting cycle, the total electrical energy consumed, TEC-H, by the vapor compression system while delivering heat following the completion of the defrosting cycle, and the total electrical energy consumed, TEC-D, by the vapor compression system during the defrosting cycle; and initiating a next defrosting cycle at a time that is based at least in part on one or more characteristics of the cumulative coefficient of performance, CCOP,, wherein the CCOP is determined by dividing the total heat delivered, THD, by the sum of the total electrical energy consumed, TEC-H, by the vapor compression system while delivering heat plus the total electrical energy consumed, TEC-D, by the vapor compression system during the defrosting cycle; and wherein the total electrical energy consumed, TEC-D, by the vapor compression system during the defrosting cycle is an average of the total electrical energy consumed, TEC-D, by the vapor compression system during a previous "N" of the defrosting cycle, wherein "N" is an integer greater than 1.