Paying more because of your postal code

It revealed that prices for gas and electricity can vary by an average of £100 according to where you live.

"While suppliers choose to target ultra-competitive online plans regionally we will see a postcode lottery. Regional pricing is a key part of the competitive battlefield," says uSwitch's Ann Robinson, right.

ScottishPower had the biggest inconsistency, charging up to £189 extra for customers living in different areas, despite their being on the same energy plan, paying in the same way and using the same amount of energy.

The most expensive place to live energy-wise is Cardiff, where online customers pay £949 a year on average, while Nottingham is the cheapest with an average of £855 for online tariffs.

This discrepancy, uSwitch says, has been commonplace on standard energy plans but is now affecting online customers. Going online is still the cheapest way to pay for energy, but, says Ms Robinson: "The key thing is for consumers to minimise the impact, by making sure they move to the most competitive plan with the most competitive supplier in their region."