Lower Temperature, Lower Costs

You can expend the swimming season by the use of a pool heater, gas or electric. However, for every degree rise you dial up, the cost in energy will be significant. Swimming in a bath is an idea that is nice, but you can swim just as enjoyably in a cooler pool, and save money in the process. If you really want to save money, look to install a solar collector system as a stand alone or adjunct to your established pool heater. It can pre heat the water and save energy.

Of course, when the pool is not in use, turn it down and use a pool blanket – cover to retain the heat generated.

More info here;

https://www.energy.gov/energysaver/heat-pump-swimming-pool-heaters