The grid has collapsed at least four times this year, which authorities blame on technical problems. Last month workers from the Transmission Company of Nigeria (TCN) went on strike and temporarily shut the grid.
Electricity distribution firms, known as Discos, said the outage happened earlier on Monday and they were working with TCN to restore supplies. They did not state the cause of the outage.
Nigeria has installed capacity of 12,500 megawatts but produces about a quarter of that, leaving many Nigerians and businesses reliant on diesel-powered generators. Diesel prices have soared since the start of the year.
The nation's sclerotic power grid, and its precarious energy supply, are often cited by businesses as a key issue hindering growth in Africa's most populous country.
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