How electric night storage heaters work
Having stored heat in special heat-retaining bricks, night storage heaters then give out heat slowly and are designed to keep warm for the whole of the following day. You can only get cheap night-time electricity if you are on an off-peak tariff such as Economy 7, Economy 10 or Warmwise, and this may involve adding a second electricity meter if you do not already have one (and paying an additional standing charge). You will need to check this with your electricity supplier.
Modern, slim-line storage heaters often have a charge control (or an automatic charge control) which adjusts the amount of heat stored overnight. An automatic charge control does this by measuring the temperature in the room (or more rarely, outside the house) and if it is milder, stores less heat (saving money in the process). If the storage heater has a manual charge control, you will have to make this adjustment yourself and keep an eye on the weather forecast.
Electric night storage heaters give out their heat in two ways:
Radiation from the front panel - this cannot be turned up or down, so will give out heat continuously, day and night (as long as there are still warm bricks behind it).
Convected heat in the form of warm air. This can be adjusted simply by flaps above the heat store, which can constrict the airflow through the store, so reducing the amount of heat taken from the store. More sophisticated storage heaters also have a fan which can blow air over the storage area to produce a heat boost. However, this will also cool the heater down more quickly and, if used too much, might result in the storage heater getting cold before the evening when it is able to be heated up again.
Many people like to take advantage of storage heaters for background heat, but find that in the middle of winter they still need to supplement the heating by a direct electric heater, such as a fan heater. Although they are no longer being marketed as aggressively as they were before the electricity companies were privatised, storage heaters are still widely available from leading manufacturers such as Creda, Sunhouse, Dimplex and Stiebel Eltron. Individual heaters are typically rated from 0.9kW to 3.5kW; the size you will need depends on the size of the room to be heated and the amount of insulation. At the top of the range, storage heaters can now offer 7-day programming to allow for differing heat profiles between working days and weekends. Although more expensive to buy, these are particularly useful in living rooms where patterns of use vary across the week, or for shift workers, but if you are on a tight budget cheaper models can often be used for bedrooms.
Further information about storage heaters is available from the Storage Heaters website.
'Wet' electric storage systems with radiators
A second type of storage system uses off-peak electricity to heat up a large, very well-insulated tank of water, typically to 85°C. This is then pumped into a traditional 'wet' central heating system with radiators, which can be controlled through a normal controls package (timer, room thermostat and TRVs on radiators). If the temperature in the thermal store falls below 60°C, the water can be kept to temperature using on-peak electricity. This sort of system is typically installed to replace a solid fuel (coal) heating system where homes are off the mains gas grid, avoiding the need for the fuel storage tanks required by oil or LPG systems, or for use in relatively small flats and apartments. Typically, there are few products currently available in the UK market. If you're interested in this technology, it's worth consulting a qualified heating engineer, but products such as the Gledhill Torrent Greenheat, while designed mainly to work with wood burning stoves, can be used in electric only mode; its Electramate is no longer available. The Belgian company ACV has also supplied some electric boiler systems for use with thermal stores, including the E-tech, but national distribution has not always been available. Heatrae Sadia also manufacturer a range of domestic electric boilers. This type of boiler is however falling in popularity as ground or air source heat pumps usually offer lower running costs, if slightly higher installation costs.
Radiator-based electric systems generally do not need additional room heaters providing the thermal store is properly sized. However, in larger homes, they are sometimes used with efficient wood-burning stoves to provide additional spot heating in living rooms.
Electric underfloor heating
Electric underfloor heating works in much the same way as storage heaters, except that there are fewer controls. In most cases, the electric heating element is laid in the floor at the time the house (or, most often, flat) is built, and concrete is poured around the heating elements to provide the thermal mass instead of the heat retaining bricks. The floor is sometimes then tiled with special heat-spreading tiles. Underfloor heating of this type can be expensive to run. It does not work well with fitted carpets. The main advantage of this type of heating is that there are no radiators or storage heaters on any walls. It is often associated with an old electric tariff called "white meter", where a completely separate meter is used for the heating system with its own rates and time clock.