Annual Fuel Utilization Efficiency, or AFUE, is a measure used to evaluate the efficiency of heating systems, particularly furnaces and boilers. It indicates how much of the fuel consumed is converted into usable heat for a home or building. The percentage reflects the amount of energy that is effectively used, with higher percentages signifying more efficient systems. For example, a heating system with an AFUE rating of 90% means that 90% of the fuel is used for heating, while the remaining 10% is lost as exhaust or waste. Understanding AFUE helps consumers choose more efficient heating options, potentially leading to lower energy bills and reduced environmental impact.