An 84-year-old war veteran tried to take his own life after a utility firm wrongly claimed he owed £6,500.
The error led to Walter Bargate, from Stockport, being left without power. He swallowed 100 pills and wrote a note naming E.On as the "catalyst".
He was found unconscious but recovered. It emerged his bills had been inflated because E.On had wrongly wired a meter.
A spokesman said E.On had corrected the fault as soon as it was made aware of it and offered "sincere apologies".
Watchdog Energywatch slammed E.On for a "catalogue of crass errors" and called for a review of how vulnerable customers are protected at a time when energy bills are soaring.
Mr Bargate - who served in Bomber Command in World War II - was found unconscious at his home in Great Moor, Stockport.
In his farewell note to his children he explained how a few days earlier engineers had entered his home and installed a pre-payment meter. After two days the meter ran out of credit and his power was cut off.
Mr Bargate, who is physically impaired and lived alone, said he topped up his account but never had his supply restored.
He then called the company for advice but said he was left hanging on for more than three hours.
As he recovered in hospital, his daughter contacted E.On who told her he owed £6,500 for energy payments accrued since 2003.
It emerged he had been unable to pay increasingly high bills - of up to £1,800 a quarter - and had hidden the debt from his family.
Following Mr Bargate's attempt to take his own life, his daughter, Sarah Hayes, contacted Energywatch and the energy supply ombudsman who launched an investigation.
The inquiry revealed the meters in his home had been incorrectly wired and he was being charged up to four times more than he should have been.
The ombudsman ruled E.On should write off the £6,500 bill and apologise to the family.
The ruling suggested that while the faulty wiring dated back to 2002 when the meters were installed, the problem could go back to 1983 when E.On took over Mr Bargate's supply.
Mrs Hayes said: "They behaved in the most atrocious manner from beginning to end.
"It chokes me to think how desperate he must have felt.
"He became afraid and stopped opening his letters."
Adam Scorer of Energywatch said: "E.On have a legal and moral obligation to acknowledge the horrendous impact of their actions on Mr Bargate's quality of life and to provide suitable redress."
A spokesperson for E.On said: "It is extremely important to us that any customer who is having difficulty paying their bills contact us as soon as possible so that we can help.
"Had we been made aware of the severity of this situation by Mr Bargate or his family we would have acted upon it immediately.
"We have a range of ways to help vulnerable customers to ensure debts do not arise and we would urge any customer experiencing difficulties with their account to call us."