Turning down my house thermostat at night is resulting in $300 plus electric bills on a 100 gallon tank here in SoCal. I'm using two 300 watt heaters, so that's 6 amps for 10-12 hours per day. Is 600 watts overkill for 100 gallons? Would it be cheaper to set the house thermostat at 70 through the night (gas heat)?