How does the ESO manage periods of low demand?

You may have heard us talking about “low demand” periods for electricity ever since the coronavirus pandemic kicked off.

Because lots of industry has temporarily shut down as we all isolate, and firms and factories use a lot of electricity, we have seen overall demand fall by as much as 20% in some cases.

If you’re wondering what this means for our system and what we do when there is too much electricity kicking around, you need to firstly understand that if lots of electricity is being generated that isn’t needed, we can’t just get rid of it, we have to balance the system.

We make sure demand equals supply and we do this second by second, 24/7. That’s because if things fall out of balance, the frequency drops, voltage drops, and we will see power cuts – so we can never allow this to happen.

So, what can we actually do if too much electricity is coming into our system?

Very simply we can either try and raise demand, so we use up some of the excess, or we try and lower supply.

To lower supply we can “constrain” generation, which means we can ask generators to stop producing electricity. Traditional coal and gas power stations can do this instantly as can some others, like wind. For some generators it isn’t as easy – we can’t stop the sun making solar energy and nuclear plants need quite a bit of notice before they can turn off. 

And some generators can’t be on “standby” connected to our system, they have to connect at a certain minimum level to operate in a safe and stable manner (a bit like a car stalling without enough gas) and the minimum levels are unique to every generator. If fewer generators can connect, the ones that can must meet all of the country’s demand. 

We can look to send our electricity to Europe via our interconnectors and this also decreases supply, but it only works under some conditions, for example if the weather is really sunny here but poor elsewhere and it’s cheaper for Europe to import our electricity.

Then we can try and increase demand. Flexible tariffs are helpful here because industrial and domestic users are paid to use increase their consumption, helping us by using up some of the excess electricity.

We have some electricity generation that is plugged straight into local distribution networks – typically this could be small wind farms or domestic solar – we can try and switch this off. But we have very limited sight of this because it doesn’t come directly onto our big national system.

We can also call on what we call “pumped storage”. This is when we pull water back up a mountain or dam – it takes lots of energy to do this so it makes sense to carry out when there is lots of excess electricity on the system. Last year we successfully trialled a project with a major water company to turn up their demand when we gave the signal – it’s an example of what we call demand side response.

The main thing to take away is that people should not worry about their supply of electricity. We have years of experience balancing our system and a whole range of tools we can deploy to keep it stable. It’s actually one person’s job to sit and watch the frequency of our system second by second, so that it never drops – that’s how seriously we take it.

Our system is one of the most dependable networks in the world, with a reliability rate of over 99.9% and we have planned long and carefully for a whole range of unexpected scenarios – including pandemics like this one.