Costing Â£4million, the 7,000 panels have been installed at the Promens plastics plant near Beccles, Suffolk.
With a capacity of 1.65MW, the panels produce enough energy to power 480 homes.
The UK’s largest solar roof installation went live in time to beat a Government deadline next week to withdraw Feed-in Tariff funding for larger sized schemes.
Adrian Banks, engineering manager at Promens, said: ‘This is really another step along the path of commitment to reduce our impact on the environment and this is the most significant step Promens Beccles has done in the last 12 months.’
Almost all of the power produced from the sun’s energy falling on the plant roof will be used on-site, with any surplus feeding into the national grid.
The scheme was paid for by hundreds of small investors from across the UK who contributed sums starting at Â£3,000.
They were brought together by fund manager Octopus to cash in on a Government subsidised scheme, that has now had its rules changed to allow individual households to benefit.
The installation was developed by Lightsource, a renewable energy installer and operator, and built by Grupotec, a Spanish engineering and architectural consultancy which specialises in green power generation.
Nicholas Boyle, chief executive of Lightsource, said: ‘This roof-top solar plant highlights the important role solar generation has to play in our future energy mix, particularly when the energy it produces is used at source, removing the need for wasteful distribution networks.’
‘Getting this site online before the August deadline was a considerable achievement and it is a real credit to Grupotec, for their speed and coordination during design and construction, and to Promens as a company for their commitment to the project and a willingness to do what was needed to get it completed.’
Paul Latham, managing director of Octopus said: ‘The Promens installation isnâ€™t just a great achievement in its own right, but also a validation that Octopus can deliver to its investors what we said we would. Given the tight deadline set by the Government, we were one of very few investment houses able to provide funding for solar projects such as this one.
‘However, none of it could have been done without the support from our investors, who recognised that solar offers a unique opportunity to benefit from Government-approved incentives by investing as little as Â£3,000.’
It is the largest initiative of its kind to date and changes to Government subsidies are expected to limit future developments on this scale.
Ministers were becoming concerned that solar farm developers would drain the limited funds that are primarily intended for domestic rooftop installations.
The Government has dramatically reduced the subsidies available for any solar scheme over 50KW coming online after August 1, so it is likely that this will be the UKâ€™s largest roof-top installation ever.