I have a 480 gallon hot tub with a 50 amp 240 volt circuit feeding it. It does a great job of increasing my carbon footprint, but an even better job at managing my stress, so there ya go. The heater can increase the temp by about 5-7 degrees per hour depending on the water temp and outside temp. When I drain it and refill it with 55 degree hose water, it takes at least 8 hours to heat back up.
I gotta wonder how long wood would take.