Very common to do in warmer climates like So. Cal. If it freezes where you are, you need to install heater in heated shed/room and the pipes will need protected also. You will probably not save any money and if you do it will take a long time to realize the savings because the heater is so expensive. The wire that went to the heating element can go to the gas heater to power it up. I once put a 225,000 btu heater on a 350 gallon tub and installed a remote control to turn it on in their house....that sucker was hot in no time. Make sure you have it passed by your local city inspector.