Staff at Leicester-based Mark Group Ltd, which was founded in 1974, were told the news on Wednesday afternoon.
Administrator Deloitte has made 939 redundancies, but a further 200 jobs are at risk unless a buyer can be found.
A statement said Mark Group had sustained heavy losses due to "structural changes in core markets".
The firm employs 1,165 people across the UK, with more than 700 based at its headquarters in Beaumont Leys.
More than 570 jobs have gone at the company's headquarters, with 154 retained while a buyer is sought.
In September last year, the firm announced it was cutting 670 jobs across the UK as a result of falling demand.
It said a change in government policy on energy efficiency had resulted in a "significant reduction" in installation work.
The Sustainable Energy Association said: "This is disastrous news for the thousand or so employees that have left Mark Group buildings to tell their families they no longer have a job."
Chief executive Dave Sowden is calling on the government to rethink its entire approach to energy policy.
He said: "This could easily have been avoided if the government had paid attention to clear signals about the need to instil confidence in the market.
"Ministers were warned long before the election of a confidence problem in the sector due to lack of clarity on policy and frankly could have easily prevented this."
A spokesperson for the Department of Energy and Climate Change said: "All job losses are regrettable and we sympathise with those affected, but commercial decisions are a matter for the company concerned.
"Our priority is to keep bills as low as possible for hardworking families and businesses.
"Government support has driven down the cost of renewable energy significantly and these costs are continuing to fall. We are protecting existing investment and bill payers, while reducing our emissions in the most cost-effective way."
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