Almost 10,000 solar panels have been imported from the United States and are placed in 12 lines in a six-acre field.
The ?2.5m investment will be onstream three weeks before the UK government lowers the subsidy for large-scale solar energy investors.
The site's owner Western Solar still hopes to double its size.
It is run by Dr Glen Peters who owns Rhosygilwen mansion and art centre with his family.
He said: "There are 10,000 panels here. They are very cutting edge from the States.
"They are thin film, particularly suited to our climate here of largely cloudy skies."
He has planning consent for a development twice the size but had to rethink his plans.
"There was no bank financing available. I then had to take a total act of faith and said 'okay, we will halve the scheme, we will do one megawatt initially' and I basically raided my pension fund."The development would be enough to power 300 homes.
Other applications for three and five megawatt solar parks at Cynheidre and Ffos Las in west Wales are said to be still in planning.
But while Rhosygilwen has beaten the government's closing of a lucrative loophole, developers like Nigel Payne of Allied Renewables in Swansea are setting their sights lower.
His company hopes to complete three 50 kilowatt solar parks, half the size of Rhosygilwen, by September.
Expansion concernAnother 10 are in the planning stage and, by reducing the size of the output, will still be able to generate a return of 30.7p per kilowatt hour.
"It spreads the feed-in tariff to what it was designed for - not supporting large-scale solar farms where subsidies would be absolutely gobbled up," he said.
The Department of Energy and Climate Change has said from 1 August tariffs would be reduced for large solar panel investors.
Any large-scale solar farms above 250 kilowatts, and up to 5 megawatts, will be able to claim 8.5p per kilowatt hour.
Schemes between 150 kilowatts and 250 kilowatts will be able to claim 15p per kilowatt hour and schemes ranging from 50 kilowatts to 150 kilowatts 19p per kilowatt hour.
Solar installations below 50 kilowatts are unchanged.
The average household installation, less than 4 kilowatts, will still be claiming the highest bracket of 43.3p per kilowatt hour.
With the solar industry increasing over the past 12 months from generating 4 megawatt of power in Britain to 96 megawatts, Dr Owen Guy, Swansea University's senior lecturer in nano technology, said there were some concerns that expansion could slow down.
"It's still available for the small-scale projects. Individuals will be able to install four kilowatt systems on their homes and will still be able to get a good return on their investment," he said.
"But the large scale companies wont be able to make the profit they have been."
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