Heat pump pool heaters are primarily electrically powered heating units that operate on similar principals as a home central air conditioning system.

An electric heat pump is essentially a reverse air conditioner for your pool. It takes ambient air heat and transfers that energy to the water. There are even pool heat pump models that can cool a pool if desired. 

An electric heat pump produces much lower amounts of heat when compared to a fossil fuels gas heater and as such are not ideal for spas or for times or climates where the ambient air temperature drops much below 50 degrees Fahrenheit. Because electrical heat pumps operate without the added costs of fossil fuels using one can often heat pools much more cost effectively than using a gas heater AND they can operate at lower temperature ranges than a solar heater. 

Often residential pool heat pumps output ranges from 95,000 to 140,000 BTUs of energy output.