Solar pool heaters are considered the most energy-efficient type of pool heater. They use energy from the sun to heat the water in your pool, which means they have zero operating costs once installed. While solar pool heaters can have a higher upfront cost than other types of heaters, they can provide significant long-term savings on energy bills. Additionally, solar heaters are environmentally friendly and can reduce your carbon footprint. Heat pumps are another energy-efficient option that uses electricity to transfer heat from the surrounding air to the pool water. Still, they typically have higher operating costs than solar heaters.