It doesn't really work like that, it heats up on with the cheap rate electric (Economy10) and is on a thermostat so it's only on for around an hour over the whole day - but that's still 3kWh!! 
Even the washer dryer combi doesn't use that much lol. I know it's not an efficient system, using electric to heat water never will be, but it's the only option for those of us without gas or oil.
Even the washer dryer combi doesn't use that much lol. I know it's not an efficient system, using electric to heat water never will be, but it's the only option for those of us without gas or oil.