The immersion tank is the hot water supply for your property. You can usually find it in a hall or bathroom cupboard.
It should look a little like this picture.
1 – Economy 7 switch – this will come on during the night and give you half price hot water.
2 – Boost switch (avoid having this on as it’s expensive)
3 – The water tank is very well insulated and will keep water hot all day.
Day time = 42 pence / hour
Overnight charge 21pence / hour
Heating water for 7 hours overnight costs 7 x 21p = £1.47
*rough estimates based on 14 pence per kilowatt