Ratio of work or useful output to the amount of work or energy input, used generally as a measure of the energy-efficiency of air conditioners, space heaters and other cooling and heating devices. COP equals heat delivered (output) in British thermal units (Btu) per hour divided by the heat equivalent of the electric energy input (one watt = 3.413 Btu/hour) or, alternatively, energy efficiency ratio divided by 3.413. Higher the COP, higher the efficiency of the equipment.
Read more: http://www.businessdictionary.com/definition/coefficient-of-performance-COP.html