Electricity meter clocks showing the wrong time are going unnoticed, meaning up to 3.9 million households on reliant tariffs could be paying too much, consumer group Which? has reported.
The watchdog said several members had come forward to warn that their meter clocks had been showing the wrong time, potentially leaving them "significantly out of pocket".
Time-of-use tariffs such as Economy 7 and Economy 10 offer customers a lower rate for electricity during some hours and a higher rate for others.
Some 3.9 million UK households are on these types of tariff.
Gary Day, a retired engineer from South Wales, told Which? he had saved himself and three neighbours more than £2,300 after discovering their meter clocks were wrong by several hours.
They were all on time-of-use tariffs with supplier Swalec, which supplies gas and electricity to customers throughout Wales.
Mr Day told Which? magazine: "I have only checked four meters and every single one of them was wrong.
"I am horrified that there might be hundreds of others that have these clock errors and don't realise it."
His neighbour Andrew Murphy claimed back around £800 after discovering his clock was about three hours out.
Mr Murphy told Which?: "The problem is most consumers don't go around checking, and we are at a disadvantage because of that."
Swalec told Which? that staff were not required to check the clocks when they read meters, although if they happened to notice any problems they must report them.
It said it could not release any information about the scale of similar problems.
Ofgem licence conditions say suppliers must inspect meters at least once every two years.
A Which? spokeswoman said: "Having a faulty clock on your energy meter could leave you hundreds of pounds out of pocket.
"It's the supplier's responsibility to ensure they are correct so if you suspect there might be a fault then contact your energy company."
A Swalec spokeswoman said: "For anyone whose meter clock is incorrect we will recalculate their charges accordingly and either fix their meter clock or if necessary replace their meter.
"We will always make sure our customers are not left out of pocket. If anyone suspects their meter clock may be incorrect, or would like us to check, please give us a call on 0800 975 8317."