Cal Spas® offers a Hybrid option for heating its swim spas. The optional upgrade from a 5.5 kilowatt electric heater to an 11 kilowatt Hybrid heater uses natural gas as well as electric power to amplify and nearly double the heating capacity of the standard electric-only heater. This translates to more power when heating the 15 to 25 hundred gallons of water in your swim spa to higher temperatures.