Fortunately, the 150,000-tonne vessel was clear of navigational hazards and drifted in open sea, the Marine Accident Investigation Branch (MAIB) said.
The emergency generator started automatically and then main generators were restarted and the ship was able to resume sailing after about an hour.
No one was injured in the incident in the early hours of September 23 2010 as the vessel was approaching Barcelona.
The accident caused extensive damage in part of the aft main switchboard room and was triggered by deterioration in the capacitors, which store energy.
''A current imbalance detection system, which was the only means to warn against capacitor deterioration, was found to be inoperable, and it was evident that it had not worked for several years,'' the report said.
The MAIB said that since the incident, action had been taken by Cunard's parent company, Carnival, and by Lloyds Register.
The MAIB also made safety recommendations to Lloyd's Register, Carnival UK and to the Maritime and Coastguard Agency.
Date: December 22, 2011 Source: telegraph.co.uk