COP or Coefficient of Performance is a term specifically used in heat transfer to describe the efficiency of any device transferring heat. All heaters are compared to a standard form of heating called resistance heating. Resistance heat is transferred by passing water over a red-hot coil (like in your hot water tank or toaster oven). This form of heating is constant and has a COP of 1 (one).
The COP of all heaters are compared to this standard, so a COP of 6.21 means that the heater is 6.21 times more efficient than resistant heating. If you compare that same heater to one with a COP of 4.14, a heater with a COP of 6.21 would be 50% more efficient. The COP of a heat pump generally measures between 5 and 6, which translates to an efficiency rate between 500% and 600%.
The higher the number the less it will cost to operate.