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
A freezing, frosty night for some of us, milder for others
Researchers are developing vaccines for new, emerging and unknown diseases before they cause global health emergencies.
Barack Obama urged Donald Trump to "go forward with his vision and his values", as he warned of "complexities" in office.