Electric [1] swimming pool heat pumps take the heat out of the outside air and put it into the pool, generally producing three to five units of usable heat energy for every unit of electricity consumed. Why don’t smaller heat pumps do well in cold weather? Colder weather requires the machines to run longer – Although manufacturer hourly heat output ratings multiplied by the longer running hours might suggest that the smaller machine can do the job, a heat pump does not produce heat at anywhere near its rated efficiency when it is forced to run during much of the colder nighttime hours so larger pools require larger [2] heat pumps.