The company said its Northern and Playford B power stations and Leigh Creek coal mine would not operate beyond March 2018 and might close even sooner.
The closures are not expected to occur before next March but were inevitable as operations had become "increasingly uneconomic", according to the company.
Alinta Energy CEO Jeff Dimery said the decision to close operations had been outlined to the workforce and key stakeholders.
He said the company was liaising closely with the South Australian Government, unions and the communities at Port Augusta and Leigh Creek about the changes.
"There are approximately 440 employees that will be affected by this difficult decision," he said.
"Their welfare and that of their families is our highest priority at this time.
"Our decision to close the business has been made at the conclusion of a detailed strategic review, during which we have consulted extensively and investigated a range of options with key stakeholders over recent months, including the SA Government."
Mr Dimery said that during more than four years of running the business, Alinta had incurred operating losses of about $100 million while also investing another $200 million to extend the operating life of the business.