Cosatu did not oppose loans in principle, as they were a legitimate source of revenue and an alternative to the tariff hikes poor households found crippling.
"We agree, however, with the National Union of Mineworkers' (NUM) call on both the government and Eskom to make public all the conditions attached to this World Bank loan."
This kind of loan often came with stringent conditions, Cosatu said.
"Indeed, the World Bank has a notorious record of using conditional loans, particularly to developing countries, to impose their neoliberal agenda and demand privatisation and opening up of markets to big, multi-national companies."
Cosatu shared the NUM's concern that the conditions attached to the loan should not open up South Africa's energy sector to global competitors and stifle the country's desire to electrify all households in South Africa.
"The federation needs to be certain that there are no conditions which could lead to any form of privatisation, including the introduction of independent power producers into the industry and ending Eskom's monopoly in electricity generation."
Should there prove to be any such strings, Cosatu would oppose the loan.
'Opposed to privatisation'
"We remain opposed to privatisation of the country's basic infrastructure.
"In Eskom's case it will inevitably lead to even higher tariffs, retrenchments and worse service, as the new owners is driven only by the need to maximise their profits."
Ordinary South Africans would be burdened for generations to come if the conditions were too onerous, added the Congress of the People.
That the ANC stood to gain financially from the loan through its links to a company granted a tender on the Medupi project was "indefensible", said spokesperson Phillip Dexter.
"It amounts to looting the public purse. It is imperative for the ANC to sever all financial ties to the project," he said.
Cope noted that, among others, the United States and the United Kingdom had abstained from decision-making on the loan.
This, because the South African government had done "almost nothing" to promote the development of alternatives to coal-fired power generation, or to find viable schemes to mitigate for the environmental impact of fossil fuels, said Dexter.
In a statement issued late on Thursday, the World Bank said its board of executive directors had approved the loan "to help South Africa achieve a reliable electricity supply while also financing some of the biggest solar and wind power plants in the developing world".
The loan was the bank's first major lending engagement with South Africa since the fall of apartheid 16 years ago.