We have been reviewing pool heaters since 2015. Recently, we did in-depth online research to determine how effectively different solar, gas and electric heaters work for both in-ground and above-ground pools. Our research revealed that the Hayward HeatPro HP20654 is the overall best heater. It is a powerful, energy-efficient in-ground pool heater that delivers 85,000 Btu with a maximum temperature allowance of 104 degrees Fahrenheit. It has a UV-protected outer shell, as well as a corrosion-resistant evaporator fin that is specially designed for longevity in coastal environments.