Storage Heaters
These operate using low cost off-peak electricity such as Economy 7 and White Meter.
Storage heaters all operate on the same principle. A core of very high density bricks store heat during the hours that the cheaper rate electricity is available then release it as and when required.
The bricks are very well insulated so that heat is retained until it’s needed. Even so, as the temperature within the core is very high there will always be a small proportion of heat continuously given up to the room.