Guernsey and Jersey suffered power failures after a fault in the French grid caused the electricity cable link to trip.
The islands' power supplies were lost shortly before 19:30 BST and some properties in Guernsey had to wait several hours to be reconnected.
Jersey was reconnected to the French grid but emergency turbines are being used on Guernsey to provide power.
Alan Bates, of Guernsey Electricity, apologised for the disruption caused.
He said: "It was caused by factors outside our control."
Mr Bates said three diesel generators were running within five minutes of the outage and the feeds out to the island were re-established by 20:50.
However, some areas remained dark for longer as faults were identified and fixed.
A joint investigation with French firm RTE and Jersey Electricity has started.
Guernsey Airport managed to avoid any disruption by making use of emergency lighting.
Mr Bates said some power was again being imported from France and the island would return to being solely fed by the cable link before lunchtime.
He said: "We are looking at the resilience of the island and trying to understand how we could become more secure by maybe going to a second location in France with second cables.
"The security of supply is one of our utmost priorities and maintaining the power station is essential."