HSPF = Heating Season Performance Factor. This is a measure of a heat pump’s efficiency over an average heating season, using heating output in BTUs compared to electricity consumed in watt-hours.
SEER = Seasonal Energy Efficiency Ratio. This is a measure of a heat pump’s efficiency over an average cooling season, using cooling output in BTUs compared to electricity consumed in watt-hours.
COP = Coefficient of Performance. This is the ratio of cooling or heating output to energy consumed at a specific temperature. This ratio converts both output and consumption to a common unit, making this different than HSPF or SEER which use a ratio of BTUs to watthours. The higher the COP, the better. The higher the rating, the more efficient the unit.