You need to have a special meter from your supplier with two readings, normal and low (economy 7)
Economy 7 gives off peak cheaper electricity for 7 hours at night. Usually two elements are provided. One at the bottom of the hot water cylinder heats the whole tank. It only operates on the cheaper rate, so is wired via a timer. The top one is used to heat part of the tank as a back up and is switched on as required. If you have storage heaters fed from a separate consumer unit wired to operate on the low rate, the bottom element can be connected to it, otherwise a timer is needed.
There can be 2 separate elements, one at the top and one at the bottom.
Sometimes both elements are contained in a single unit mounted at the top of the cylinder, one is longer than the other and reaches further into the cylinder.
If there is only on element, it can be connected via a control unit that switches it on for the "off peak" electricity and has an override for top up during the day if required.
