Tanzania has introduced daytime power cuts, after drought left hydroelectric plants short of water.
Tanzania produces only half of its energy needs
Water levels in the main hydroelectric source are only 59cm (2 feet) above the level at which production would have to halt, the energy minister said.
The minister, Ibrahim Msabaha, said electricity would be off from 0800 to about 1700 each day.
Failed rains across East Africa have left some 11m people needing food aid in Somalia, Kenya and Ethiopia.
The BBC's Vicky Ntetema in Dar es Salaam says power has been off since this morning, and the noise of generators can be heard throughout the city.
She says people have accepted the cuts as inevitable given the drought, and religious leaders are being urged to pray for rain.
Electricity cuts have become increasingly frequent in the last two years as the water level in the dams gradually fell, our correspondent adds.
"We are now forced to use water at Mtera Dam [the main hydroelectric source] frugally until mid-March, when an additional 60-megawatt gas turbine will be installed at Ubungo Power Station in Dar es Salaam," Mr Msabaha told the AFP news agency.
Tanzania Electric Supply Company (Tanesco) is currently producing only half of national needs, the minister added.
"Under such circumstances, power rationing is now inevitable," Mr Msabaha said.
The minister added that power would be on at night.
"It is important to have light at night to curb crime," he explained.