Both Live Earth and the BBC's now-cancelled awareness day, Planet Relief, wanted a mass electricity "switch off". But would that work?
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It's a classic from the world of sci-fi. An aerial view of the lights going off across a city usually shows aliens are in the neighbourhood.
And it's the kind of dramatic scene that the impresarios behind the Live Earth concerts and the producers of the BBC's now-cancelled Planet Relief were hoping for when they conceived a big "switch off". A grand gesture - a couple of minutes with the lights and appliances off - to show people that they could save energy and save the planet.
Switching off at the same time would cause problems - potentially loss of supply or wasted energy
But the National Grid was nervous over the idea, advising against Live Earth's switch off plan and negotiating for a year with the BBC on an alternative for Planet Relief.
Their reasoning was that the unpredictability of demand during such an event could mean some people losing their electricity supply and even raise the danger of emitting more carbon dioxide rather than less. While a sudden disappearance of demand is an unusual thing for the National Grid, its engineers know a lot about surges.
The UK has for many years, and more so than many other industrial nations, sporadically come together to watch the same television programmes in vast numbers.
You can imagine the scene. Emile Heskey is stepping up to take the decisive penalty in the Euro 2008 semi-final shootout against Portugal. The ball rolls gently towards the post and bounces away to the collective groans of a nation.
There's shuffling on 10 million sofas a third of the population mope out to the kitchen. Click. Millions of lights and kettles are simultaneously switched on as the traumatised nation seeks solace in a cup of tea. This sudden surge in demand for electricity is known by those in the industry as a "TV pick-up". And it must be prepared for.
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The multi-channel era is having an impact on such episodes of mass viewing, but there remain occasions like major soap storylines, football matches and royal weddings that have vast appeal.
The National Grid faces a constant challenge. It must provide enough energy, and not too much, and it must keep the frequency at 50hz.
Alan Smart, energy operations manager for National Grid, conjures up a Speed-like analogy.
"The way to think about it is to imagine you are in your car and your challenge is to keep the car at exactly 50 miles an hour. You press on the accelerator as you go up the hill, and you ease off on the other side.
"We can't store electricity in any great quantity, so we have to forecast second by second, minute by minute. You base that on what did it do yesterday, what did it do last week, can you identify a day with exactly the same weather."
There is a demand-forecasting computer program that looks at the corresponding five weeks over the past five years. And better still there is an analyst who tackles the TV listings every day and tries to predict the spikes.
"There will be somebody now looking at this evening's television schedules and forecasting what the control room should expect."
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Typical evening demand this week across England, Scotland and Wales will be in the regions of 41,000 MW.
"A TV pick-up will give you anything between 200-400 [extra MW] if it's not a major storyline; for a main character being killed or a wedding with a lot of hype 700-800." If the analysts' predictions are more than 300MW out, the incident might be investigated.
For changes in generation the National Grid needs at least two minutes' notice, so the key is in getting its predictions right. It has back-up generators, but these are to cope for contingencies like a power station suddenly shutting down due to malfunction.
But the problem for the idea of a big switch-off is that there aren't very many examples of sudden decreases in demand triggered by TV programmes. A rare example would be the two minutes' silence at 11am on Remembrance Sunday, but this occurs during daylight hours so is nowhere near comparable.
The crunch issue with the big switch-off idea, Mr Smart suggests, is that nobody would be able to estimate how many people would take part. Estimate too few and loss of supply could follow. Too many and unnecessary energy could be wasted.
"Saying we want to switch off everything now is not going to be sustainable and is very, very unpredictable.
You would create far more carbon [emission] than what you would actually save."
Instead, for the ill-starred Planet Relief, the compromise solution, after a year of negotiation, was that viewers would be asked to conserve energy in the 24 hours before the programme and the National Grid would provide an up-to-date analysis of how much in emissions had been avoided.
But for the moment the idea of a dramatic switch-off is, well, switched off.