Hundreds of thousands of Npower gas customers who were victims of a massive billing blunder are still to reclaim £20 million combined.

Payments for overcharging that dates back to 2007 are up to £117.50 per person at an average of £41, plus interest (see the Cheap Gas & Electricity guide).

However, only 1.2 million of the 1.89 million who received refunds notices have grabbed their money, leaving almost 700,000 still to claim.

The gas refunds total £70 million but only £51 million has been recovered so far, according to lobby group Consumer Focus, whose pressure led to the refunds.

How to get a refund

The energy giant has sent refund forms to households but these must be cashed in at a Post Office branch within six months of the date of issue.

Christine Farnish, from Consumer Focus, says: "We want to encourage all those consumers still entitled to a refund to get the cash that's rightfully theirs.

"We urge anyone with a payment letter at home from Npower to take it to their local Post Office now and get their refund."

Former Npower customer who's moved home?

Npower has written to all existing customers and to the last known address for all former customers.

This means some ex-customers will need to contact the energy giant if they've moved home, to notify of their new address.

If that's you, then call Npower's helpline on 0800 975 7938.

The following customers were not overcharged: electricity customers; business customers; gas pre-payment meter customers; anyone on the Gas Guardian tariff; 2009 Price Fix tariff; Tracker Tariff; 1st steps (social tariff) or the Sign Online 8 tariff.

How did the blunder happen?

The billing problems centre on the fact Npower has two tiers of charges, like most energy firms. You are charged more for the first set of units used in a period than when you reach the second tier, at which point the price per unit drops.

In this case, too much of many households' usage was deemed to be in the higher band.

Further reading/Key links

Slash energy costs: Cheap Gas & Electricity
Your rights: Fight Energy Direct Debits Boiler protection: Boiler Cover