The most efficient way to heat a pool is using a solar cover, in combination with an energy-efficient gas, heat pump, or solar pool heater.

Energy.gov states:

“You can improve your swimming pool’s heating and energy efficiency by installing an energy efficient pool heater [gas, heat pump, or solar] and taking the steps to reduce pool heating costs.” (4)

In terms of overall efficiency, though, solar covers could be considered superior. After all, they require less energy (and utility bills) than other heat sources.