Modern heat pumps are efficient for both heating and cooling operation. Cooling efficiency is measured by SEER, or the Season Energy Efficiency Ratio. This is the ratio between the cooling output of an AC system seasonally divided by the energy consumption of the system. The higher the SEER rating the more efficient the system is. Lennox makes AC systems with SEER ratings from 13.0 to 26.0. Mitsubishi mono=split systems offer SEER ratings from 12.8 to 33.1. Heat efficiency is measured in HSPF (Heating Seasonal Performance Factor) for air source heat pumps. This is the ratio of seasonal heating output divided by the energy input of the system. The higher the HSPF the more efficient the system is from heating. Lennox split heat pumps offer HSPF ratings from 8.2 to 10.2. Mitsubishi offers heat pumps with HSPF ratings from 8.5 to 13.0