Annual Fuel Utilization Efficiency (AFUE) is the measurement that indicates how high efficiency a furnace is. AFUE is calculated by dividing an appliance's yearly heat output by its yearly fuel consumption over a set time period, usually one year. The higher the percentage of efficiency, the more energy saving your high efficiency furnace. AFUE ratings range from 0 percent to 100 percent, with higher amounts indicating greater efficiency in converting fuel into useful heat.