When it comes to keeping your swimming pool temperature right, and sourcing the best, most economical or most environmentally friendly heater, they type of pool you own matters. The above ground pool heaters are not quite the same as the in ground ones.
Buy why is this? Surely they both use the same type of water... In fact you need a more powerful heater to keep the temperature up for an out of the ground pool than one with identical dimensions that is planted in the ground.
The reason is fairly simple - above grounds pools generally require 30% more heat put in to them because of the additional heat loss through the sides of the pool. In other words the ground is a better insulator than the air.
That said, there is no problem finding a suitable heater for either type of pool. There are solar swimming pool heaters for above ground pools.
Above ground propane pool heaters need to be bought specifically for one or the other type of pool. Of course there are electric swimming pool heaters - traditional or heat pump available for both.
To make a difference to your heating bills and help the environment, anyone with an above ground model should look to install an R7 rated insulating layer on the outer walls of the pool. It will make a big difference.