economy 7

Economy 7 is the name of a differential tariff provided by United Kingdom electricity suppliers that uses base load generation to provide cheap off-peak electricity during the night.
Houses using the Economy 7 tariff require a special electricity meter which provides two different readings - one for electricity used during the day, priced higher, and the other for the night, priced lower. The night (off-peak) period lasts for a total of seven hours, hence the name; however it may not be a continuous period, as it may alternate between the two prices during the night.
The term was coined by Jon Marshall. The first mention of Economy 7 is in 1978:

A new off-peak tariff known as the 'Economy 7' tariff was introduced in October [1978]. It featured a seven-hour night rate some 20 per cent cheaper than most night-time tariffs, made possible by economies in the night-time operation of the system.
In more recent years the difference between day and night rates has become smaller, with a reduction of about 33%
(though dependent on the supplier). The Economy 7 tariff results in either or both of an increased standing (fixed) charge or increased daytime rate.

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