COP and EER

COP (Coefficient of Performance) and EER (Energy Efficiency Ratio) measure how much heating or cooling a unit delivers per unit of electricity, higher is better, and 3.5+ is good.

A COP of 4 means 4kW of heating from 1kW of electricity, the apparent magic of heat pumps moving heat rather than creating it. EER is the cooling equivalent. Seasonal versions (ACOP/AEER) average performance across a whole year and are the fairer comparison. Two otherwise similar quotes can hide a meaningful efficiency gap here; ten summers of power bills will find it.