When we turn it on, the heaters come on and stay on for over an hour. This would be similar to what happens in a hot water heater.
True, and in an area where a kilowatt costs the same at 9:00 AM as at 3:00 AM, there is no savings in using energy at one time versus another. Boston is not such a place, however. The cost of a kilowatt depends on when you use it. So, while it might take twice as much energy to reheat the water (probably not, is the heater is well insulated), it can be cheaper overall to heat the water when energy is cheaper.