Thousands of gas customers could be due a refund from energy companies after big six supplier E.on spotted a major flaw in the way some meters are being read.

The problem relates to some measurements from old-style imperial meters being confused with modern metric ones, and vice versa.

In some cases customers could be due £10,000+ after paying almost triple what they should have done for up to 15 years. In others, customers were only charged a third of what they should have paid.

E.on says at least 600 of its customers – both homes and businesses – are affected, and has warned it's discovered a "potentially industry-wide meter set-up issue".

Energy regulator Ofgem now says it's investigating the scale of the wider problem, raising the prospect that thousands of energy users with other suppliers could also have been affected.

E.on says those found to have been overcharged will be refunded, while others who've been undercharged won't be expected to stump up the difference.

What went wrong?

This billing issue arose because of a mix-up between imperial meters, showing usage in cubic feet, and metric ones, showing usage in cubic meters.

In 350 cases, E.on customers' metric meters were read as imperial, resulting in them being overcharged by 183%, and in 250 cases imperial meters were read as metric, resulting in them being undercharged by 65%.

E.on told us: "This issue goes back a significant amount of time, for some around 15 years, however the majority [of customers] were affected for much less than this. Each case will be dealt with on an individual basis."

How do I know if I'm affected?

Ofgem says there's no need to do anything as anyone who's been affected will be contacted by their energy company.

But if you're unsure, you can first of all find out whether your gas meter is based on metric or imperial measurements just by looking at it:

  • An imperial meter will usually say "ft³" somewhere on the front of it, while a metric one will normally say "m³".

  • With a metric meter partial units of gas are highlighted in red and those digits are often separated from the other numbers by a decimal point. With an imperial meter the partial units are often displayed on a dial. See the website for pictures which show these differences.

At this stage only E.on is known to have made the error and only 0.03% of its customers have been incorrectly charged.

What if I've been incorrectly charged?

If you have been wrongly billed, you should be told by your supplier.

E.on says it's giving anyone overcharged a full refund, and that those who've been undercharged won't have to pay anything extra. Refunds are being dealt with on a case-by-case basis but are likely to be given in the form of a cheque.

We don't yet know for certain if customers of other firms are affected – and so there's no news yet on what'll happen if they have been. But Ofgem says it is "working with suppliers to ensure consumers don't lose out". A spokesperson added: "This includes suppliers setting out plans to redress their affected customers, and to reach a quick resolution".

How many people does this affect?

We don't yet know for sure how wide a problem this could be. E.on says the 600 customers it's identified represents 0.03% of its users, so if a similar proportion are affected at other firms it could be thousands.

But Ofgem says it's not yet clear if that's the case. It's given other energy suppliers until Friday to confirm whether they've made the same metering mistake as E.on, and we'll update this story when we know more.

What does Ofgem say?

An Ofgem spokesperson says: "Ofgem is aware that a very small number of gas customers are being incorrectly charged for their energy use and has taken fast action. Working with [trade association] Energy UK, we're ensuring suppliers take immediate and decisive action to identify affected customers.

"Customers do not need to do anything. The very few affected will be contacted by their supplier."