All heating and cooling systems have a rated efficiency from a U.S. governmental agency. Fossil fuel furnaces have a percentage efficiency rating. Natural gas, propane and fuel oil furnaces have efficiency ratings based on laboratory conditions. To get an accurate installed efficiency rating, factors such as fuel gas heat losses and cycling losses caused by over sizing, blower fan electrical usage, etc., must be included. Geothermal heat pumps, as well as all other types of heat pumps, have efficiencies rated according to their coefficient of performance or COP. It's a scientific way of determining how much energy the system produces versus how much it uses. Most geothermal heat pump systems have COPs of 3-4.5 (Water Furnace's E Series is rated up to 4.7). That means for every unit of energy used to power the system, 3-4.5 units are supplied as heat. Where a fossil fuel furnace may be 78-90 percent efficient, a geothermal heat pump is about 400 percent efficient. Some geothermal heat pump manufacturers and electric utilities use computers to accurately determine the operating efficiency of a system for your home or building.