25 fire engines and about 100 firefighters were needed to tackle a blaze at Didcot B, Oxfordshire, which broke out at about eight o'clock on Sunday evening.
Energy Secretary Ed Davey said the National Grid have assured him that there was no risk to energy supplies.
Mr Davey said: "My priority is to understand the cause of the fire and get the affected unit back, generating electricity as soon as it's safe to do so."
The fire was extinguished after shutting down part of the site, and there were no injuries.
The fire is not being treated as arson or a terrorist incident.
