Npower, the second-most complained-about energy firm in Britain, is writing off debts and providing free energy to customers worst-affected by its chaotic billing system.

It said 1,000 customers so far will benefit from the move. They have been waiting for at least four months – and in some cases as long as 20 months – to have their billing complaint resolved, according to The Telegraph.

Npower agreed the concession after talks with the ombudsman and the energy regulator, Ofgem, which deemed such lengthy waits unacceptable.

In each case, Npower itself failed to resolve the customer’s complaint within eight weeks. After this time, householders can take their case to the Energy Ombudsman, an independent dispute resolution service, which gives Npower 28 days to resolve the issue.

All the 1,000 cases to be offered compensation had filled their complaint to the Ombudsman. Many go back as far as December 2013, Npower confirmed.

“We are recognizing this particular group has been subject to delays”, a spokesman said. Npower added that more customers than the initial 1,000 could be given free energy “where a remedy has not been completed due to our process or system”, but that this would be reviewed on a case-by-case basis.