How Much Can a High-Efficiency Furnace Save on Your Energy Bills? Furnace efficiency is calculated according to Annual Fuel Utilization Efficiency — or AFUE — ratings. This rating is expressed as a percentage that tells you how much of the fuel you are using in the furnace is being transferred into heat energy for your home. So, if your furnace has an AFUE of 75%, this means 25% of the energy in your fuel is […]