The immediate cause of the change in risk is the reduction in thermal power generation arising from the closure of a number of older power stations. The effect of changing the generation mix is to reduce the de-rated marginal capacity: the electrical generation capacity of the system, de-rated to refl ect the availability of plant, less the peak electricity demand. This margin is forecast to decline from in excess of 15% to below 4% by 2015 to 2016. As this margin decreases, so the risk of brownouts and blackouts increases.

The UK has a very reliable electricity generation and supply network. However, the likelihood of a short voltage reduction (a brownout) or a controlled shutdown (a blackout) will increase signifi cantly, reaching a maximum around 2015–2016, when more than one-third (35%) of businesses could be affected by a blackout.