A EER unit is a type of energy efficiency rating used to measure the efficiency of air conditioners and heat pumps. It stands for Energy Efficiency Ratio and is calculated by dividing the cooling output of a unit by its energy input. This ratio is expressed as a number between 1 and 10, with 10 being the most efficient. A higher EER rating indicates that the unit is more efficient and will use less energy to cool the same amount of space. It is important to consider the EER rating when purchasing an air conditioner or heat pump as it can have a significant impact on energy costs. The higher the EER rating, the more money you can save on energy bills.