Around 2,000 people are relying on generators to provide electricity after an undersea power cable failed.
The 55km cable is the only connection between the Isles of Scilly and the National Grid.
It failed on Monday and it could be at least a further ten days before it is fixed.
Western Power Distribution (WPD) said the fault is 17km from Land's End. The generators were operational within two minutes of the cable failing.
WPD has chartered a specialist ship from Global Marine Systems to carry out the repairs.
The CS Sovereign will travel from Portland, Dorset, on Wednesday, at which point WPD hopes the repairs will be completed within a week.
A remotely-operated underwater vehicle (ROV) will be deployed to the seabed to cut the cable, which will then be lifted to the surface for repairs.
John Nesbitt, distribution manager for West Cornwall with WPD, said: "There's a constant fuel supply for the generators. This was always the contingency plan and it's working well."
It's not known what has caused the damage to the cable.