Mainstream Renewable Power lost €66 million last year, figures filed by the Irish energy business show.
The company, which develops wind-powered electricity generators around the world, had revenues of €5.65 million in 2020, against €70.7 million the previous year.
Mainstream sold its Neart na Gaoithe wind farm project in Scotland for €56.6 million in 2019, boosting the group's revenues and profit.
Accounts filed with the Companies Registration Office show it lost €65.85 million before tax in 2020, against a profit of €19.36 million the previous year.
A €300,000 tax bill lifted the loss for last year to €66.1 million, compared with an overall €19 million gain in 2019.
Eddie O'Connor, Mainstream's founder, resigned as executive chairman in May following "inappropriate and insensitive" remarks about doing business in Africa.
Earlier this year, Aker Horizons, part of Norwegian billionaire Kjell Inge Rokke's fishing-to-engineering Aker empire, paid an initial €900 million for a 75 per cent stake in Mainstream.
The Scandinavian player could pay a further €100 million in 2023 if the Irish business hits certain performance targets.
Mainstream’s accounts show the energy company earned revenues of €575,000 from generating and selling electricity last year, from zero in 2019.
Fees of €2.73 million for construction management provided to projects in its Aela Energía joint venture in Chile, and developments in South Africa where it is a minority owner, were the single biggest source of revenue.
Along with the reduced revenue, costs rose last year. Administration expenses were €39.44 million from €24.77 million in 2019.
Mainstream boosted overall job numbers by 12 to 278, paying them €17.5 million in wages, social security and pensions. Directors received €3.5 million in total.
Chief executive Mary Quaneystates that the company is still on track for a possible stock exchange launch in two or three years.
"Aker Horizons listed on the Euronext Growth Index in Oslo shortly after its agreement to invest in Mainstream in quarter one [of] 2021 and a number of Aker Horizon's businesses have already listed on the Oslo stock exchange," she said.
She added that Aker’s backing positions Mainstream to cash in on growing renewable energy opportunities.
Mainstream is building wind farms with a total capacity of 1,200 mega watts (MW) in Europe, Africa and Latin America. It aims to complete planning and funding for a further 5,500MW by 2023.