The problem with electricity is that it does not come out of the ground like gas, it must be generated, and not only that, it cannot be stored so must be generated as it is needed. (Batteries do not store electricity, they convert it into chemical energy, and this process is at present very inefficient and very expensive and uses potentially very dangerous chemicals, as anyone who has needed to replace the battery in his iPad® or had her mobile phone explode will testify.) But the demand for electricity varies hour by hour, day by day, and month by month, so if we provide enough generating capacity to meet the maximum demand there will be times when there will be a lot of spare capacity. So we have a pricing structure which is designed to encourage people to use less electricity at peak times and more at off-peak times. So far so good.
But there are some processes which use large amounts of electricity but which do not need to use it all the time. Users can be offered a much lower rate for electricity for these processes if they agree to use electricity only at times of low demand - an interruptible rate, where the supply is actually turned off except at times of low demand. This price can be set very low because it allows users to buy large amounts of energy which would otherwise, in effect, be thrown away.
The more the country switches to wind farms, where the time the wind is best for generating electricity is totally unrelated to the demand for it, and nuclear power stations, where there is no advantage of any sort in operating at less than 100% capacity, the lower the price of interruptible energy can be.
Interruptible rates have been around for a long time: sixty years ago when I was at University studying aeronautical engineering we had a supersonic wind tunnel which was powered by air stored at very high pressure in huge tanks. The air pumps needed to fill the tanks used so much electricity that if we had turned them on at, say, 17:30 on a Friday evening in December we would have blacked out a large part of South Kensington! So we had an interruptible rate, set very low because we were using electricity which no one else wanted. Today interruptible rates are used by, for example, cold-storage companies, water desalination plants and companies making hydrogen from water as fuel for cars and aeroplanes.
Some, but sadly not all, energy companies offer interruptible rates to domestic customers with storage heaters, even more sadly interruptible rates and storage heaters are not mentioned at all in any of the Government literature about smart meters and how you can save money by switching on your immersion only at off-peak times - yet another example of the ignorance of most people about storage heating.