Pool heaters can extend the swimming season well past summer. Swimming pool heaters are powered by either electricity or gas. Often confused with pool heater pumps, pool heaters warm up a pool a lot faster and keep it warm for longer. Swimming pool heaters have thermostats that can be set to the precise environmental conditions. Most pool heaters units are available in powers from 75,000 BTUs to 400,000 BTUs - the higher the BTUs, the quicker the water heats up. The size and power of the heater needed will depend on the amount of water in the pool. Above ground pool heaters are cheaper, starting at $650, and inground heaters will start at $900 online.