Immersion heaters are an efficient way to heat your domestic hot water to your desired temperature. However, the problem is the heater element automatically turns on once the water temperature drops below the temperature you've set. If your house is empty during the day you don't need hot water until the evening -- and similarly for night and morning -- then your immersion heater turning on and off is an expensive waste of electricity. Immersion heaters generally only take about half an hour to heat the water so you can wire an timer into the circuitry, allowing it to turn on and off at your preset times.
Purchase a plug-in timer adaptor from an electrical store. It's the simplest method to control the times your immersion heater turns on and off; and it's probably the least expensive. You can purchase analogue timers, or the slightly more expensive digital timers; both are widely available.
Remove the plug on the end of the wire from the immersion heater from the wall socket. Plug the timer into the wall socket.
Set the correct time on the timer. Set the times you want your immersion heater to turn on and off. The timer includes an instruction leaflet on how to preset times.
Insert the plug on the end of the wire from the immersion heater into the timer adaptor socket. It's usually on the front or top of the timer and is the same type of socket as your wall socket. Your immersion heater will turn on and off automatically at the preset times.