Thousands of Ovo and Good Energy customers are to get refunds after the regulator found that the firms had overcharged households.
Ofgem has ordered the energy firms to pay a total of £4million - £2.7million of which will be split between 18,000 homes.
The average repayment to affected Ovo customers will be £181, while Good Energy will refund £109.
The regulator said yesterday the suppliers had charged people above the rate allowed under two energy price caps.
Dan Norton, deputy director of retail at Ofgem, told the BBC: "It is totally unacceptable that Good Energy and Ovo Energy customers were overcharged, particularly at a time that is already so challenging and stressful for consumers across the UK."
Those customers who have been affected will be automatically refunded.
Vulnerable
The remaining £1.25million will be paid solely by Good Energy into Ofgem's voluntary-redress fund, which aims to help "consumers in a vulnerable situation across the UK".
It was the second time in two days that Good Energy had been punished by Ofgem.
On Wednesday, Good Energy, alongside E.On Next and Octopus Energy, were told to pay a total of £8million for delaying or failing to make compensation payments to customers who had switched supplier.
Ofgem said that Good Energy and the other two companies had either missed or "unduly delayed" compensation payments "which are due if a supplier does not provide a final bill within six weeks when a customer switches to another provider".
On May 25, Ofgem will announce its latest price cap.
FTSE 100
The UK's top share index, the FTSE 100, was up 20 points at 7,763 shortly after opening this morning, following yesterday's 19-point gain.
Brent crude futures were 0.98% higher at $76.60 a barrel.
Companies reporting today
- Trading update: Smiths Group