Kevin Walsh, managing director for Renewable Energy GE, which built the project, said the unit was expected to have the highest capacity of any solar-energy project in the world, but a power plant in Germany had overtaken it.
He said: "But as far as we know - thanks to great Portuguese sunshine and high technology - this plant right here in Serpa is expected to produce the most power - more than 20 gigawatt-hours per year."
Surge in emissions
The plant, which has 52,000 photovoltaic modules, is near the town of Serpa, southeast of Lisbon.
The scheme fits into Portugal's plans of reducing its reliance on imported energy and cutting output of greenhouse gasses that feed global warming.
Portugal's emissions have surged by about 37 per cent since 1990, one of the highest increases in the world.
By bringing modern technology to one of western Europe's poorest regions, the $75m plant is expected to bring alternative development to the Alentejo.
There are also plans to build a solar-power plant in the neighbouring town of Moura.