When it comes to swiming pool heaters you have plenty of options. The first option, chosen by many users, is to have no heating! Maybe some users are surprised by the cost of pool heaters (they can cost as much as some pools) and elect to go without. This is a shame however because no matter how hot your summer is, the season for an unheated pool will be very short.
The cheapest option as far as ongoing costs are concerned is probably solar heating for swimming pools. This is also the most environmentally friendly. It is even possible to work the pool pump from solar power, thereby meaning that the pool costs hardly anything to run. Good for you and the environment.
You may really want to stick with electricity. After all, it is convenient because most homes already have electricity supplies, and it is usually fairly compact compared to solar panels. You can choose the most energy efficient option these days - many swimming pool heat pumps are on the market that make the best use of our electricity.
A perennial favorite are the propane pool heaters or those versions that run from natural gas. These have been around for a long while and many current users swear by them. The good news is that they are better than ever - gas heaters have really come on in terms of efficiency over the last decade or so.
There are other options worth considering. An environmentally sound option may be one of the new ultra-efficient wood burning pool heaters, for example. It is well worth shopping around for a fuel that makes heating your pool cheap and convenient.