E.ON is paying out a total of £3.1 million after missing appointments with customers and then failing to compensate them.
Ofgem said the energy giant had already paid £1.2 million to affected customers and would pay a further £1.9 million to charities for falling "well short" of consumer standards.
The regulator said E.ON's agents missed appointments as required by the Guaranteed Standards, which require suppliers to meet minimum requirements including when they need to visit customers' premises.
The company told Ofgem in 2014 that it had missed some appointments with customers and had not made amends by paying compensation.
Ofgem said it had worked with the company to agree a redress package rather than take formal enforcement action.
It said the £1.9 million going to charity includes helping service personnel through National Energy Action's Help for Heroes scheme.
Ofgem chief executive Dermot Nolan said: "E.ON fell well short of the high standards we expect for consumers when it missed appointments and then failed to compensate customers.
"It's crucial that suppliers provide their customers with a fast and effective service, and make amends when things go wrong."