The performance efficiency of any heating or cooling system can be stated in terms of COP and EER.
What are they?
COP is a ratio number describing the energy efficiency of a HEATING system.
EER is a ratio number describing the energy efficiency of a COOLING system.
Both COP and EER are determined by comparing the amount of heating and cooling generated per1kW of electrical power used. For example, an air conditioner that generates 3.6kW of heating when using an electrical consumption of 1kW has a COP of 3.6. Likewise for cooling the EER would be 3.6.
So, when trying to compare relative efficiency of two units then look at eh COP and/or EER values - the higher the number the more efficient it will be. (The only problem with this system is that a lot of manufacturers do not yet produce this information!)