By Richard Black
Environment correspondent, BBC News website
The event hopes to make people aware of how energy is wasted
Energy Saving Day, a 24-hour initiative aiming to reduce the UK's electricity use, begins on Wednesday evening.
A coalition of environmental groups, religious leaders and energy companies is asking people to curb climate change by turning off devices not in use.
The National Grid will monitor how much difference it makes to consumption, while power companies will identify customers wanting home insulation.
The BBC News website will be displaying results in close to real time.
Organisations from all sectors of society have been prepared to see what they can do to help tackle climate change
Energy Saving Day, or E-Day, will be launched on the steps of St Paul's Cathedral in central London at 1800GMT.
The Bishop of London is due to speak, and the event will feature a bicycle-powered cinema showing short films relating to climate change.
"I'm delighted by the way in which so many organisations from all sectors of society have been prepared to see what they can do to help tackle climate change," said Matt Prescott, a long-time campaigner for low-energy lightbulbs and the E-Day originator.
"They have offered to set aside their day-to-day differences in order to highlight that they accept the available science, agree that saving energy is a good idea, and want to simplify and widen access to some of the other potential solutions to climate change."
The National Grid will be able to see if the campaign affects demand
Those organisations include some of the UK's principal energy retailers, and the environmental groups that sometimes lambast them for their greenhouse gas emissions.
"We do call on companies to take more action," said Ashok Sinha, director of Stop Climate Chaos, an umbrella campaigning group on climate change.
"But we think it's welcome that they're encouraging their customers to save energy - that's responsible behaviour for an energy company."
The government obliges energy providers to offer energy saving measures to their customers.
People can use the E-Day website to register their interest in receiving help from these companies with loft and wall insulation.
E-Day started life as a Planet Relief, which was to have been an awareness-raising BBC TV programme with a large element of comedy.
But in September the BBC decided to pull the project, saying viewers preferred factual or documentary programmes about climate change.
The decision came after poor audiences for Live Earth, and public debate over whether it was the corporation's role to "save the planet".
This data is supplied by the National Grid, via E-Day. It is updated every 30 minutes during the 24 hours of E-Day.
Actual consumption shows the energy used so far during the 24 hours; Usual consumption is what the National Grid predicted under "business as usual".
Dr Prescott then decided to see whether he could mount E-Day as an independent operation, and secured the backing of important partners such as the energy companies and the National Grid.
Its role is crucial, acting as an independent and credible monitor of how much difference E-Day makes to the UK's electricity consumption.
Part of the Grid's job is to forecast demand for electricity. It says its forecasts are usually accurate to within 1% - so comparing demand across the 24 hours of E-Day with its predictions should provide an accurate measure of whether the initiative has made much difference.