What is the best and most cost efficient way to heat a swimming pool?

This is a vague question! What type of pool are you heating? You have a lot of options, as you probably already know because you are asking for ''the best'' way. Solar heating is the cheapest way, but isn't very effective, in my opinion. If you decide to purchase an elecric or gas heater, you must know what you are looking for to determine what is going to be the best for you. When buying a pool heater: what type of pool are you heating (in or above ground), what is your rate of flow, and how much water do you need to heat.