An arsonist is thought to have pushed a bin against a Tesco building before lighting it on fire.
The bin fire, at Tesco in Middlefields, Letchworth, spread to an electricity store, which caused power to fail in the supermarket and building damage.
A Hertfordshire police spokesperson said the fire broke out at around 12.45am on Wednesday, April 27.
Nobody was hurt, but officers investigating the suspected arson have launched an appeal for witnesses.
Detective Constable Nick Jenkins, from the North Herts Local Crime Unit, said: "Thankfully, no one appears to have been injured during the incident, but it has resulted in damage to the building and stock.
"We are appealing for anyone with information to please come forward.
"Did you witness any suspicious activity in the area around the time?
"Did you capture anything of note on dash cam?"
Anyone with information can file a report to police online (https://www.herts.police.uk/), or phone 101, quoting crime reference 41/32942/22.
Alternatively, anonymous reports can be submitted to CrimeStoppers online (https://crimestoppers-uk.org/) or by phone: 0800 555 111.
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