Thursday, 23 October 2014

Will the lights stay on over the winter? Comparing capacity and demand.

There have been a number of unplanned power station shutdowns recently for various reasons - bringing the prospect of black outs, or maybe brown outs, over the winter ever nearer. How close are we? I have been looking at the figures for plant capacity and power demand, to see how tight the situation really is.

Electricity demand varies from year to year, mainly due to weather. The highest demand we have ever seen was 60.1 GW in December 2002. However last winter, the peak was 53.4 MW (25th Nov 2013) and in the previous winter the peak was 56.3 GW [1].

In terms of capacity, adding up all the weather-independent power (fossil fuel plant, biomass burning and nuclear) as of the end of 2013 comes to 71.7 GW.

Also, depending on the weather we had another 1.5 GW of hydro power and up to 4.8 GW of wind power available. Since then we have even more wind capacity. In fact on Saturday 18th October at one point we were getting 6.4 GW from wind [2]. If we have a windy winter we will have no problems at all with power supply. However, let's ignore wind for the moment and consider a worst case scenario.

Since the end of 2013, we have had some planned shutdowns and some unplanned ones, as shown in the table.

StationMW capacityTypeReason
Ferrybridge C 1 and 2
planned termination
Ferrybridge C 3 and 4
planned termination
Didcot B
Heysham 1 and Hartlepool

Some of these stations may be fixed before the winter sets in, but let's assume they aren't for the moment. That brings the total weather-independent capacity down to 66.0 GW - 10% more than the highest demand ever and 18% more than the highest demand in the last two years. That sounds pretty comfortable.

However, there is another important factor. Even weather-independent power stations are not always available. They do need maintenance - some more than others. Nuclear power stations tend to run when they can but last year they only generated 74% of their theoretical capacity. Gas power stations tend to be more reliable, usually available 90% of the time or more, assuming no problems with gas supply but that looks OK for now. To be safe, maybe we should assume that only 85% of capacity is actually available. That brings the 66 MW down to 56.1 GW - just a teensy bit less than the 2012 peak of 56.3 GW. Oops.

Also, we can expect that when margins are that tight the wholesale market price will be high, so we will have to pay more.

On the other hand, there were only three half-hour periods that winter when demand was greater than 56.1 GW [3], so if there were power cuts they would not be for long.

In any case, it is likely that industrial customers on Short Term Operating Reserve contracts will take the hit by reducing their demand (see The cost of keeping the lights on at peak times). Also, we can reduce peak load at home by, for example, not running the washing machine, tumble dryer or dishwasher between 5 and 7pm. If everyone did this it could reduce demand by 1-2 GW (see How to avoid brown-outs this winter).

Also, the chances that there aren't any wind power are small. Over the last two years or so there has been at least 0.5 GW of wind for 90% of the time and 70% of the time we have more than 1 GW [4].

Plus we can buy power from Europe via interconnects with France (2 GW), Netherlands (1 GW) and Ireland (1.1 GW) if they have any spare.

In summary:

  • At the end of 2013 we had 71.7 GW
  • Various shutdowns have reduced this to 66.0 GW
  • Assuming 15% downtime due to maintenance, reduces this to 56.1 GW
  • This would have easily covered demand 2013/2014 (53.4 GW). It would have been tight in 2012/2013 (56.3 GW) - we could have had power cuts of perhaps 2 hours.
  • But 90% of the time we have wind power of at least 0.5 GW
  • And we can import up to 4.1 GW from abroad.
If the worst came to the worst:
  • Industrial customers on STOR contracts could take the hit
  • And we can do stuff at home to reduce demand by another 1 GW or more.

So I think we are very probably going to be fine.

[1] Digest UK Energy Statistics 2014 (DECC)
[2] Wind Farm Outstrips Nuclear Power (BBC) 22nd Oct.
[3] Historic data downloaded from
[4] Historic data downloaded from

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