Electricity is a commodity that cannot be stored economically in large quantities. Therefore, generation must equal demand plus energy lost as heat when electricity is transported (transmission / grid losses) on an instantaneous basis. Otherwise the grid frequency starts deviating from its reference value, which can result in a system collapse. The design of electricity markets is adapted to deal with this particular property of electricity. Different types of electricity markets are arranged in a sequential order, starting years before the actual delivery and ending after the actual delivery.