Up to 45,000 households in south-western Berlin could be without power until January 8 after a fire on a Teltow Canal bridge.
Grid company Stromnetz Berlin confirmed the outage, which occurred early on Saturday.
Police are investigating the incident, which damaged high-voltage cables near the Lichterfelde heat and power station, as a possible arson attack.
“We expect a full restart of all customers by Thursday afternoon,” the company said in a statement, noting it would require the installation of new cables.
It said some 2,200 commercial entities were also affected and mobile phone services and landlines might also be impacted.
Firefighters had been alerted at 0545 GMT and put out the fire, according to a police note posted on Twitter.
Criminal investigators and emergency services are on the scene, the police said.