Energy-saving cooking bags for sale at the 2022 Mill Road Winter Fair

Want to save energy this winter? A Cambridge woman who volunteers with The Children’s Society will be selling cook bags at Saturday’s Mill Road Winter Fair that act like a slow cooker – but without using any electricity. Instead, they rely on basic heat retention.

Gillian Stephens, who has been making the bags herself to sell outside the charity’s shop, explains: “You treat it as a slow cooker but there’s no energy used. There’s no battery, there’s no gas, there’s no electricity.

“You may remember our grandmothers used hay boxes – it’s the same principle of thermal heat. You bring your stew up to the boil on the cooker, then you put the saucepan in the bag, leave it and serve it. So there are just four steps: boil it, bag it, cook it, serve it.”

Gillian makes her bags using donated fabrics, pillows, blankets and duvets, using her sewing machine at home. “I’ve done a pumpkin and then a more upright one, like a barrel, and a square,” she says. “I found a pattern on Etsy for the barrel-type ones, and there are a lot of YouTube videos on how to make them so I just copied them.”

Each bag takes Gillian around six to eight hours to make on her kitchen table, and she calls it a “labour of love”. “The house has turned into a factory,” she admits, “because I’ve got an awful lot of beanbag beans piled up in the conservatory, which get everywhere, and then the sewing machine’s on my table and the sitting room has got 15 bags stacked up in it!”

She hopes that the idea will catch on and that more people will start using the bags as another way of saving energy. But you can’t reserve one. “You must come to the Mill Road Winter Fair!” says Gillian. “As people don’t know what they are, I will be there with saucepans demonstrating, but I won’t have hot food there – you have to have a food licence for that.”

The large bags will be sold outside The Children’s Society shop at 258 Mill Road for £39.99, while the medium bags will be priced at £35.99 and the small bags cost £29.99.

Gillian was aware of the concept of cook bags from their popularity in South Africa, where she has cousins. “South Africa has a terrible lot of electricity outages – and as we are heading for power cuts, I thought this is the way to beat the power cut,” she said.

In 2008, Sarah Collins created Wonderbag, a UK business and South African not-for-profit, inspired by a method her grandmother used in the 1970s to help save power when cooking, and similar to the concept of a hay box used to keep food warm and preserve fuel during the Second World War.

She first tested her idea by surrounding a pan of hot food with cushions, after turning off the stove, and discovered the food kept cooking. The concept was developed to help families cook hot daily meals even when the power went off.

Now a recipe for social change, Wonderbag has created jobs in South Africa, given time back for girls to go to school and saved families from spending money buying charcoal or other fuel.

[Read more: 16 practical ways to save money on your energy bills]

If you miss out on Gillian’s cook bags on sale in Mill Road on Saturday, Wonderbags sell for £62.99 online at wonderbagworld.com.