Supermarket chain Sainsbury's will give away one million energy-saving lightbulbs at its stores on Saturday.
Sainsbury's hopes to see old-style bulbs replaced with greener ones
Timed for when the clocks go back, Sainsbury's said it wanted to encourage people to save electricity during the dark months ahead.
To claim their bulb, customers will have to take an energy-saving pledge that urges people to recycle bags and switch to showers from baths.
Sainsbury's typically sells an average of one million green lightbulbs a year.
The chain will also be urging customers to unplug their mobile phone chargers, dry clothes outside instead of in tumble dryers and keep the fridge and freezer running more efficiently.
Sainsbury's will be handing out Philips 11W energy saver bulbs, equivalent to a regular 60 watt bulb, that usually retail for £2.
Switching to just one energy-saving lightbulb, which uses 80% less energy than traditional bulbs, saves in the region of £7 every year.
Environment Secretary Hilary Benn last month unveiled plans to phase out the sale of traditional lightbulbs by 2011 under a voluntary scheme.