An immersion heater is an electric water heater that sits inside a hot-water cylinder. It acts very much like a kettle, using an electric resistance heater (which looks like a metal loop or coil) to heat the surrounding water. They can be easily switched on and off, as there's no need to constantly heat the water in your hot-water cylinder. Immersion heaters can either be used as a property's primary water heater, or as a backup water heater for combi boilers.
The cheapest way to use an immersion is to put it on about an hour before you need it. Make sure your immersion heater wall switch has a thermostatic control which switches it off when it gets to the right temperature or a boost button that will switch off after 1 hour. This might be enough to heat your water for your needs, taking about 45mins to get to temperature in summer and about 70mins in winter. Alternatively, you could set the timer for the heater to come on in the off peak early hours when the price of electricity is lowest and go off an hour or so later so its still warm for your morning shower. Make sure it is well insulated. You can buy an insulation jacket for about £20. Not only can a jacket keep water hot for several hours after it switches off, but it can save about £70 a year in energy costs.
Heating water using electricity is more expensive than heating water with gas. A typical immersion heater uses three kilowatts of electricity an hour, so it will cost the average house about 50p an hour to run. Most family households will need to run an immersion heater for at least a couple of hours a day to get the water hot enough - costing at least £360 a year.
Never leave your immersion heater on 24/7. This can be hugely expensive unless it has a thermostatic control. You need to heat the water in your immersion heater to above 50°C to kill off bacteria, but lowering it to a few degrees to a comfortable temperature above 50°C will make a considerable difference over the year.