High-efficiency systems use less energy to produce the same amount of heat or cool air, which can result in significant savings on your energy bills. Look for systems with a high SEER (Seasonal Energy Efficiency Ratio) rating for AC units and a high AFUE (Annual Fuel Utilization Efficiency) rating for furnaces.

To summarize, we generally advise that you choose a higher-efficiency furnace or A/C (at least 95% AFUE for furnaces and at least 16 SEER for air conditioners), but there tends to be diminishing returns from a purely efficiency & operating cost standpoint.