Electricity customers in Northern Ireland will save £50m (€57m) a year as part of a shake-up threatening almost 300 jobs.
The relatively old coal-fired Kilroot plant in Co Antrim faces closure in May with the loss of 240 posts after its owners confirmed it missed out on a contract to produce electricity for an island-wide wholesale market.
A further 30 jobs are threatened by a lost deal at neighbouring gas-fired Ballylumford following a supply auction which unions branded disastrous for workers.
The systems operator responsible for ensuring supply said it was confident sufficient and secure energy would be provided at the lowest possible cost.
Utility regulator Jenny Pyper said the auction was a significant milestone.
She added: "Sufficient capacity has been secured through the auction to ensure security of supply in Northern Ireland."
The total of megawatts available far exceeded peak demand, she said.
"We are concerned not only with security of supply but also that consumers only pay for generation that is actually required.
"Indeed, the auction outcome ensures that there will be savings of around £50 million per annum for Northern Ireland electricity consumers."
Unite the union represents many of those whose jobs may go.
Regional officer Joanne McWilliams said: "This decision effectively shuts down 36% of Northern Ireland’s electrical generation capacity.
"Such an outcome will be a disaster for local jobs and security of supply."
Three unions representing workers at the plants said 270 jobs were at risk.
Meanwhile, North Antrim DUP MP Ian Paisley said the Northern Ireland Affairs Committee of MPs is to investigate issues surrounding the proposed closure of Kilroot.
GMB, Prospect and Unite said 120 staff were facing redundancy at Kilroot and 30 at Ballylumford, with 120 contract workers also set to lose their jobs.
Ms McWilliams said she was shocked and angered.
"Union representatives are calling on local politicians and the UK Government to intervene in the decision of the all-island regulatory authorities and transmission system operators not to award Capacity Market Remuneration (CRM) contracts to the units."
Kilroot’s owners, AES, confirmed the two generation units at Kilroot are expected to close when the single electricity market commences on May 23, with the Ballylumford generation unit expected to close on December 31.
AES UK & Ireland president Ian Luney said: "With the likely absence of any significant and reliable new generation in the next four years and a North-South interconnector that isn’t expected to come online until 2021, we are concerned that the removal of capacity at Kilroot and Ballylumford could contribute to a significant risk to the security and stability of supply in Northern Ireland."
Assembly member Stewart Dickson expressed concern.
"Kilroot is a key part of the East Antrim community and the wider Northern Ireland economy," he said.
"The decisions around the energy auction seem premature and potentially counter-productive."