kay.... storage heater has a thermostat inside, so if the water is hot already, the electricity current will not flow, when the water colls, then the current will flow and heat up the water again... so the process goes on and on like this when you forget to switch off the heater.... electricity is ex... but this process don't really consume much elec as the water is hot to begin with, so it only takes a while to heat the water in the tank compared to heating up cold water in a full tank....