Npower has agreed to pay £3.5m in compensation to vulnerable customers that may have been on the receiving end of its aggressive sales policy.
According to the UK energy regulator Ofgem, Npower breached energy sales rules over the phone and on the doorsteps of affected customers.
"Npower has done the right thing by stepping forward and recognising that, whilst it was making changes to improve its sales processes, weaknesses remained which affected consumers' ability to compare supplier offers fairly," said Sarah Harrison, Ofgem's senior partner in charge of enforcement.
Ofgem said that its decision reflected tougher rules it had set in 2009 which Npower had not followed and it concluded these sales breaches only stopped in September 2012.
Npower will make a payment of at least £25 ($40.85, €29.9) to each of its customers who are a core group of Warm Home Discount payment recipients, according to the regulator.
Compensation not paid through the £3.5m fund would be paid through Npower's Heath Through Warmth Crisis Fund.
Npower failed to ensure that price comparisons were fair and the information some customers would receive through direct debit discounts was inaccurate, according to Ofgem.
Recently Ofgem announced that it was giving the National Grid greater powers to reward businesses that cut their electricity use at peak times.
These proposals are meant to come forward the winter of 2014/2015.