The outage is due to last several days, a government official said.
Supplies of diesel ran out at al Zahrani and Deir Ammar power stations, with energy production dropping below 200 megawatts.
Lebanon has been hit by a fuel crisis due to one of the world’s worst ever economic crashes, compounded by corruption and bad governance.
At the same time food and fuel prices have rocketed.
Residents were already battling frequent blackouts, with mains power regularly only coming on for a few hours a day before back up generators had to kick in. But a diesel shortage has often meant these too do not operate smoothly.
Last month the United Nations promised $10million to help fight the fuel crisis.
A government official said the state electricity company would try to use the army’s fuel oil reserve to operate the power plants temporarily, but that would not happen anytime soon.