asked the Secretary of State for Energy what is his Department's estimate of the balance of benefits in energy and cost terms of switching properly lagged immersion heaters on and off as opposed to leaving them on.

The benefit to the consumer in switching off a properly lagged hot water cylinder with a thermostatically controlled electric immersion heater depends on many factors such as the size of the cylinder and the temperature of the water stored. Generally with normal-sized cylinders and storage temperatures, switching off the heater overnight should save about one unit of electricity. The cost of this varies throughout the country.