Big six energy provider Npower is under fire from customers who've received huge bills months, and in some cases over a year, since they left the firm. has seen a raft of complaints on social media and on our forum about households who've switched away from Npower thinking everything was in order, only to receive bills months later due to Npower's own disgraceful failings to get bills out on time.

Some existing Npower customers have also reported bills arriving late. (Join our free Cheap Energy Club to see if you can switch and save.)

One former customer tweeted us saying she'd left Npower last June and had only recently received a bill chasing over £1,000. Another said he'd received a £350 bill 16 months after he'd left. creator Martin Lewis says: "Npower is punishing people for its own mistakes. Once people have closed an account that should be it.

"With so many people still stretched, to come back many months later and say 'oops, here's your final bill, you actually owe us a fortune and we want it now' is out of order.

"It must do the decent thing – wipe or at least reduce bills to a manageable amount, set up an easy repayment plan, and ensure it puts no one into fuel poverty.

"If not, then let's hope the regulator has the cojones to investigate and force it to."

Npower isn't listening

However Npower, the UK's most complained-about energy provider in 2013 according to charity Citizens Advice, isn't listening. It says its "back-billing" policy only writes off charges over 12 months old for customers who have experienced issues that are its fault.

Under regulator Ofgem's rules, when you switch, your old supplier has to make "all reasonable effort" to close your account within six weeks. It should then send you a final statement showing anything that's owed.

Npower says the issue relates to customers receiving their final bills after the six-week period set out by Ofgem.

It says anyone worried about paying a bill should get in touch, and it will try to find solutions, which can include setting up a repayment plan.

'Left Npower June 2013, recently got bill chasing £1k'

After we first discovered this issue, we asked about it on Twitter. We then began to uncover a growing catalogue of Npower's failings.

  • @j40x tweeted: "We've put in a complaint. Left Npower June 2013, recently got bill chasing £1,000+. At least it can't cut us off."
  • @potnmooo tweeted: "Wow, thought it was just us. Just received a bill for £216! Left it six months ago!"
  • @snifferphillips tweeted: "Left a year ago. Got a bill for £78 couple of weeks ago! Spent almost an hour on hold trying to talk to someone to find out why."
  • @finnigan1971 tweeted: "I just got a £350 bill from Npower 16 months after I left."
  • @karldukes tweeted: "Took Npower from November until April to get a final bill once I left."
  • Forum user Baggieboy commented: "Left Npower in November. Until yesterday [3 May 2014] it still hadn't finalised my bill. It finally calculated that it owes me just over £60."

Have you been affected?  

If you've been affected, please let us know either via our forum, or by tweeting us. We want to build a case to push the regulator or Npower into action.

Why is Npower sending bills late?

The issue of Npower sending bills late first came to light in December 2013. It was told to pay £1m to vulnerable households after energy regulator Ofgem became "increasingly concerned" about the firm's consumer complaint levels.

At the time, the big six provider blamed its billing issues on switching to a new computer system, which meant a number of bills and statements didn't go out when they should have done.

Five months on, the bills are still arriving late. Npower admits that issues following the switch to its new computer system are still ongoing.

It says: "We wrote to all our customers during this period last year apologising for the impact on them of issues we have had with the implementation of our new billing system.

"We are making good progress in dealing with the root causes of this, but remain totally committed to resolving any problems this has created for our customers."

Is Ofgem going to investigate?

Last December, Npower agreed to provide Ofgem with additional regular reports on service levels. It also set out a recovery plan to ensure it improved.

Ofgem says it is monitoring Npower's progress and that it's keeping the need for any further action under review.

But hopes your complaints alongside our calls will push the regulator into taking further action over Npower's ongoing failings.

When should I receive energy bills?

Npower says "the majority" of its existing customers receive bills quarterly. But when you switch supplier, under Ofgem rules the old supplier has to make "all reasonable effort" to close your account within six weeks and send a final statement showing anything that is owed.

Where a firm doesn't do this, as in Npower's case, Ofgem can investigate.

I've received a bill late. Do I have to pay it?

Under regulator Ofgem's code of practice, suppliers can't bill customers for energy used more than a year ago, as long the reason for not paying in the first place is the supplier's fault.

Each case is looked at on its own merits, but an example of where the 12-month limit may apply is where your supplier has failed to bill you even though you've asked it to do so.

So if you've been billed by Npower for energy used over a year ago, and you asked the firm for a bill, you should complain now. See our information below on how to do so.

This is only a code of practice, so firms don't have to follow this, but Ofgem says the industry does keep to it, although it seems Npower hasn't in this case. See its back-billing guide for more information.

How do I complain about my provider?

If you're unhappy with how Npower has treated you, or if you've been hounded for bills you don't believe you're liable to pay, you should complain to Npower in the first instance. See Npower's complaints page for how.

If it fails to get back to you, or doesn't resolve your complaint to your satisfaction within eight weeks, you can take your gripe to the Ombudsman Services.

I'm owed credit by my provider. How long does it have to refund me?

As well as Npower landing former customers with hefty bills, we've also seen some reports where customers say they're actually owed money by the firm as their account is in credit.

There's no set rule on when a supplier has to return cash to customers who are in credit. Npower will automatically refund cash to customers who have an in-credit balance of £60 or more, although this will fall to £5 or more later this year.

Ofgem says it has also called on energy suppliers to do everything within their powers to return credit balances on closed accounts to customers. If Npower hasn't returned your cash, you should complain using the steps detailed above.