By Dominic Rees
Millions of households could be out of pocket after a clock error was discovered on some electricity meters.
Almost four million households use time-of-use tariffs, which see the price of electricity varied depending on the time of day it is used by consumers.
Many households struggling to make ends meet have started using the service as it provides an element of control over how much they spend on energy.
However, Which? warned that customers could be paying too much following reports that some people had found their meter clocks had been showing the wrong time.
The error could mean that customers believing they were using electricity during times when the tariff was cheaper may actually have been using energy during the more expensive period.
Time-of-use tariffs, which include include Economy 7 and Economy 10, are used in some 3.9 million UK households.
One man who discovered an error was retired engineer Gary Day, from South Wales, whose home was supplied by Swalec.
He told Which? he had saved himself and three neighbours more than £2,300 after noticing their meter clocks were wrong by several hours.
He said: "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."
'Most consumers don't go around checking'
His neighbour, Andrew Murphy, said the clock on his meter was about three hours out.
Mr Murphy, who was able to claim back around £800 after discovering the fault, said: "The problem is most consumers don't go around checking, and we are at a disadvantage because of that."
Swalec said it could not release information about the scale of similar problems.
It told Which? that its staff were not required to check the clocks when they read meters, but must report problems if they notice them when visiting homes.
The consumer organisation said suppliers have a responsibility to ensure meters are working correctly, with licence conditions stipulating they must be inspected at least once every two years.