It is the second time it’s been activated, the first was last night, after fears that electricity supply margins are tighter than usual.

On Monday, the first ever session took place between 5pm and 6pm and it saw those that reduced their electricity usage saving money.

Tonight, the service will run between 4.30pm and 6pm. The National Grid said: “As part of cautious measures to ensure we continue to have adequate operational reserves tomorrow evening, the ESO has activated the Demand Flexibility Service for Tuesday evening.”

These households will receive a discount on their energy bill, but only those with a smart meter who have signed up to the scheme can benefit.

How does the demand flexibility service work?

The service was launched in November to ease pressure on the electricity grid in particularly cold weather when demand is high. The aim is to reduce energy black outs, although these are the worst case scenario.

The first live event was due to run in November, over a fear of a drop in the energy available from France, but didn’t happen because it was not needed.

Energy suppliers sign up to the scheme and some, such as OVO and Octopus energy, have also run their own trials alongside the National Grid scheme.

Then, those customers who are signed up are told when the event will happen and during that time they are required to reduce their electricity usage, in return for a discount on their bills.

The exact amount people can save varies but some experts have estimated it could be up to £100, over the entire winter period.

Why has the service been triggered twice?

The current cold weather is putting pressure on the system as more people are keeping their energy and other electrical appliances on for longer.

As well as activating the new service, three coal plants had been asked to warm up in case they were needed to boost the UK’s electricity supply. Yet these events can be called off at any time if the weather improves.

Who can sign up to the service?

The service isn’t for everyone and you need to have a smart meter to sign up. Your energy supplier also needs to have joined, and 26 have already done so.

Then, your provider will have contacted you to join if you are eligible to ask if you want to take part in the event.

Providers have different rules around who can join, and many aren’t offering the event up to all customers, just those that have been selected. If you were selected and agreed to join, you will have been told in advance when the event would take place.