The $7.9 million project, funded by the New Zealand government, will meet up to 5 percent of the main island of Tongatapu's electricity needs.
Tonga, with a population just more than 104,500, has long been dependent on diesel generators for power, with the country consuming about 8 million gallons of diesel a year, an average of one quart every 2 seconds.
But the fuel, which must be imported, has become more expensive with rising oil prices.
Located southeast of the capital Nuku'alofa, the facility is expected to save Tonga 124,000 gallons of diesel annually while providing about 1 megawatt of electricity per year.
In 2011, electricity costs accounted for 9.5 percent of the Tonga's gross domestic product, the government says. With the new solar farm, electricity prices are expected to decrease 5.79 percent in the coming months.
As part of its "Energy Roadmap 2010-20," Tonga aims to extract half of its power from renewable sources. The solar farm is the island state's first renewable project.
''The ultimate objective is for the people to access an energy source that is clean, safe and cheap,'' said Tongan Minister for Public Enterprises William Clive Edwards, reports New Zealand's Fairfax News.
Called Maama Mai -- "Let There be Light" -- the solar farm is a partnership between Tonga Power Limited and the government of Tonga, with New Zealand's Meridian Energy serving as lead developer.
Construction on the project started in November and finished on time and under budget, the government says.
"The project has been very important to Meridian," said Meridian Chief Executive Officer Mark Binns, who was among a delegation, led by New Zealand Minister for Foreign Affairs and Trade Murray McCully, attending Tuesday's opening ceremony.
Binns said the project "has proven our expertise as a solar developer in the Pacific, setting a strong foundation for future solar developments, for when the time is right to bring the technology closer to home in Australasia."
"Tonga can be proud to be at the forefront of solar generation in the Pacific and can look forward to a better renewable energy future," Binns said.
By switching to renewable energy from diesel, said Tonga Power Chief Executive John van Brink, the island nation would realize an increase in power consumption, allowing some Tongans the opportunity to afford freezers and air conditioning.