AFUE  Annual Fuel Utilization Efficiencythe standard measurement of efficiency for gas and oilfired furnaces. Given in percentages, this number tells you how much of your fuel is being wasted. The higher the AFUE, the greater the efficiency. If a furnace has an AFUE of 80%, for every $1.00 worth of fuel, $0.80 of it is turned into heat, the other $0.20 is wasted (it's expelled as flue gas).
HSPF  Heating Seasonal Performance Factorthe efficiency measurement used to gauge the efficiency of the heating mode of heat pumps. The greater the number, the greater the efficiency of the unit. The HSPF can be thought of as the "averaged factor" for an entire heating season. HSPF can be calculated by taking the total annual heating requirements, including all energy inputs (defrost and backup heat energy included) divided by the total electric power used.
SEER  Seasonal Energy Efficiency Ratiothe measure of efficiency by which the cooling process of air conditioners and heat pumps is rated. The higher the SEER number, the greater the efficiency, which yields greater energy savings. The SEER is defined as the total cooling output (in British thermal units or Btu) provided by the unit during it's normal annual usage period divided by it's total energy input (in watthours) during the same period.
