Alternatively they may set the input control to minimum at night instead of the output, which can mean there is no heat at all for the next day. Sizing a storage heater is a compromise between the maximum expected cold-spell intensity and duration, and the cost and space requirement of the heater. If the heater is too large, its cost will be excessive and it will impact on the building's available area; if too small, the cost of supplemental (daytime) electrical heating will be excessive. Storage heaters are very heavy and somewhat bulky, due to the material used to store heat. They typically take up more floor space than the radiators used in most other heating systems. In most countries, electricity costs 2-3 times as much as gas or oil per unit of heat (joule or kWh), and the reduced off-peak tariffs and higher delivery efficiency do not fully overcome this difference. When the fan (if included) is used to speed up heat transfer, the air circulation will increase the amount of airborne dust in the heated room. This can be a problem for people with allergies. Therefore, central heating is a better source of heat for allergy sufferers. This can be avoided by not using the fan, but then it is more difficult to regulate the heat transfer, which occurs by radiation and natural convection as with normal central heating radiators. Using storage heaters Storage heaters can be cost-effective if used properly, but control may be trickier than fuel-fired systems. Power switches Storage heaters generally require two power circuits, one for on-peak and one for off-peak electricity, and two power switches, which are switched off during the summer when heat is not required. During other months the off-peak switch can be left on at all times, with the on-peak switch being used when insufficient energy has been stored during off-peak times. The amount of heat that is stored can be altered using the controls on the storage heater unit. Normally the on-peak will have a fuse as it is part of another circuit. The off-peak will just be a switch as it has a dedicated circuit. Some installations work on off-peak electricity only and cannot be switched on during the day. Basic controls Basic storage heaters have an input switch, and an output switch, called heat boost on some models. The position of the input switch may be changed