Problems with billing systems can "turn households budgets upside down" and energy companies owe it to their customers to make any changes as smoothly as possible.
Citizens Advice chief executive Gillian Guy said:
The knock-on effect of poor billing systems can turn household budgets upside down. Many people do not have the spare cash to cover the cost of a large bill that suddenly lands on their doorstep.
While we recognise Npower is receiving more complaints because it is starting to get over some of the earlier issues, it needs to do more to stop consumers' problems escalating.
Offering repayment plans and discussing ways they can help consumers from the off will nip issues in the bud and remove the need to complain.
More top news
Report criticises ministers for lack of long term plan to recruit and retain teachers in important subjects such as physics and maths.
Emergency services rushed to the scene to douse the flames, but there were no immediate reports of casualties.
A motion to remove John Bercow from his position as Speaker of the House of Commons is struggling to find support.