Since the end of the Second World War most people in Europe and North America have put petrol or diesel into their motor cars, and many of them have heated their homes and their water and cooked their food with gas, but for most of their other energy needs, at home, at school, at work, at leisure or on holiday, they have used electricity. Electricity is produced by power stations and wind farms etc.
The problem with electrical energy is that although it cannot (at present) be stored in the amounts needed to power a whole country but must be produced as it is needed the demand for electricity is not constant, so for example Britain is using a lot less electricity at 03:00 on a Saturday morning in June than it is at 17:30 on a Friday evening in December. But if there is not to be a nation-wide power cut at 17:30 on a Friday evening in December the country must have enough power stations and wind farms etc to meet this demand for electricity, that is, it must have enough generating capacity to meet not the average but the maximum demand.
Going one stage further, any change in demand must be instantly catered for, so that there isn’t a national power cut when The Big Football Match on tv ends and half the country gets up to put the kettle on.
Until the 1950s Britain got almost all of its electricity from a number of coal-fired power stations. The problem with coal-fired power stations is that you cannot just switch them on and off, you must keep the furnaces going and the water boiling and the turbines turning all the time, and this is very inefficient. A coal fired power station operating at 50% capacity uses a lot more than 50% of the coal (and therefore produces a lot more than 50% of the greenhouse gases) that it does when it is operating at 100% capacity. What was needed was a way of encouraging people to use less electricity at peak times and more at off-peak times. The obvious solution was to charge less for electricity at off-peak times. Enter electric storage heaters! An electric storage heater uses cheap off-peak electricity to store heat in special bricks which then release their heat slowly over the rest of the day. Because at the time electric storage heaters were first introduced the demand for electricity was at its lowest during the night, and therefore this was the time when electricity was cheapest, they were commonly called night storage heaters.
Since the 1950s both the way we generate electricity and the way we use it have changed a great deal, but it is still true, and will always remain true, that the demand for electricity will vary from hour to hour, day to day and month to month, and therefore if we have enough capacity to meet the maximum demand there will always be times when capacity exceeds demand. Because modern storage heaters can use electricity at any time when capacity exceeds demand, and not only at night, today they are usually referred to as just electric storage heaters, without the night and often without the electric.