Kenya Power said on Twitter some parts of the grid were already back online. It did not say what had caused the blackout.
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Blackouts occur regularly in Kenya, partly because of an ageing energy network and insufficient generation capacity. Many businesses in Nairobi and other big towns own generators as backups. On Tuesday, a deafening drone of generators could be heard in parts of Nairobi.
"We are having a national blackout," Kenya Power said on Twitter. "Restoration is in progress and we have managed to restore Western parts of the country."
Areas where power was restored included West Kenya, North and South Rift and Nairobi South, the company said.
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