Electricity is the most efficient but usually the most expensive source of heat for cooking. For this reason electric range manufacturers have proposed various methods of saving electricity; using utensils which fit the surface units, using the oven to capacity, preparing as much of the meal as possible in the insulated surface cooker, switching off the units before the end of the cooking process, and, more recently, starting oven cookery in a cold oven; in other words, beginning to use the heat in the oven when the unit is first turned on.