It was not immediately clear how Eskom, which is mired in a financial crisis and has struggled to keep the lights on over the past year, will fund the purchases.
In the announcement, Mantashe said the electricity "must be purchased by Eskom".
The official announcement of the energy allocations also paves the way for opening of a long-awaited "Bid Round 5" under the government's ambitious but delayed Renewable Energy IPP (Independent Power Producers) Procurement Programme.
"6,800 megawatts (MW) should be procured to be generated from renewable energy sources (PV and Wind)... for the years 2022 to 2024," Mantashe said.
PV stands for photo-voltaic and refers to solar power.
The statement said the 3,000 MW would be purchased from gas-generated power and 1,500 MW will be coal-generated.
($1 = 16.9240 rand)
($1 = 17.0316 rand)
(Reporting by Mfuneko Toyana; Editing by Promit Mukherjee and Emelia Sithole-Matarise)