More than 50,000 properties lost power on Saturday night when water topped flood defences at Lancaster’s main substation.
Electricity North West said engineers had been battling the damage caused by Storm Desmond since it became safe to do so on Sunday morning.
Almost 3,000 buildings remained without power across Cumbria on Monday morning.
Electricity North West operations director Mark Williamson said: “Our engineers have done an incredible job and worked around the clock to restore supplies to almost 55,000 customers in Lancaster.
“All customers have been restored from our main substation, but 300 customers in the surrounding area are still without power due to localised flooding.
“While we still have a significant amount of work to do to fully repair the substation we are still on site to repair the damage.
“I’d like to thank customers for their understanding and their goodwill to our teams during what continues to be an enormously challenging time for everyone involved. We are immensely proud of the area and the people we serve in the way they have dealt with this incident.”