Minister of Energy Kanja Sesay told Reuters that the outstanding amount "was accrued over time because the government subsidizes more than half the cost the ship charges per kilowatt hour."
He said the government had to spend more on the subsidy because it charges consumers in the weak local Leone currency, one of worst performing against the dollar in which it pays the power provider.
A government commission has been set up to review consumer electricity tariffs which could double.
Karpowership, one of the world's largest operators of floating power plants and part of the Karadeniz Energy Group, signed deals in 2018 and 2020 to provide electricity to Sierra Leone's state power utility.
The company has made similar deals with several Africa countries that are struggling with electricity supply.
Karpowership could not be reached for comment.
The company says on its website that it had deployed around 65 megawatt power generation capacity to Sierra Leone since 2020 and has been supplying 80% of its total electricity needs.
Sesay said the switch-off by Karpowership had reduced electricity supply to the capital by 13%. Electricity is now being rationed in the capital with homes and business going without electricity for hours daily.
Karpower is one of three sources of electricity to the city — the other two includes the country's hydro dam, and power from the from an interconnection with Ivory Coast which also supplies Guinea and Liberia.
Sesay said Karpower supply is mostly needed during the dry season when water levels at its dam are low. Dependence on the firm is reduced during the rainy season. The country is currently at the peak of its May to November rainy season.