Solar pool heaters are by far the least expensive to operate on an ongoing basis. Their energy cost is $0 per month. They do however incur costs on installation, maintenance/repairs and they do require attachments to roofs which can introduce potential roof leaks and their associated repair costs, or the installation of rail systems on land taking up large areas of yards. Additionally, leaking solar heater panels can drain pools if no one is home to observe a leak in progress.
Solar heaters are well fitted to be used in warmer climates as they typically top out at being able to heat a pool 18 degrees above ambient air temperature. This means that if you want to keep a pool at 85 degrees Fahrenheit then the lowest ambient air temperature which this type of heater can accommodate is: 67 degrees Fahrenheit.
This 18 degree max heating capacity is also why solar heaters are not used to heat spas under normal circumstances and why many pools that use solar heaters also have a secondary heating source to activate during those parts of the year where the temperature drops below the solar heater’s functional heating range.