Zambia has started rationing electricity supply to mining firms following reduced power generation after a big drop in water levels in lake Kariba, the chairman of state-owned power utility Zesco said on Tuesday.

"We requested them to give away 180 MW but after negotiations we went down to 110 MW," the utility's chairman Vickson Ncube told Reuters, referring to mining companies in Africa's No. 2 copper producer.

Water levels in the lake were down at 1.66% of usable storage on Monday for the Kariba North Bank Power Station in Zambia and the Kariba South Bank Power Station on the Zimbabwean side of the lake, said the Zambezi River Authority, which manages the dam.

Water levels in the lake have fallen due to reduced inflows from the Zambezi river and its tributaries and heavy use by power generation companies in Zimbabwe and Zambia.

The north bank power station at the Kariba Dam has an installed capacity of 1,080 megawatts (MW), while the south bank power station in Zimbabwe has a capacity of 1,050 MW.

Hydropower contributes to more than 75% of Zambia's electricity generation.

Ncube said power rationing was expected to be reduced by the middle of next month as water levels increased and full generation was to likely resume in March.

Last week, Zesco doubled the number of hours it cut supply to domestic customers to 12 hours from six hours daily as the low water levels in the lake threatened power generation.