Who should register? Anyone with a mandatory half-hourly meter. A half-hourly meter measures your electricity consumption every half hour, as the name suggests, sends the data back to your supplier and your supply is purchased at the price ruling on the half-hourly market, which changes throughout the day according to demand. You have to have a mandatory half-hourly meter if you have a peak load of 100kW or more.
You should know if you have a half-hourly meter: letters have been sent out to everyone who has one. Of course in a large organisation the letter may not have got to the right person, and there may be more than one meter, so make sure you track them all. If you don’t register and you should have, there are penalties.
Find more information here