The Renewable Energy Association's (REA) annual report revealed the sector grew 2.5% in 2015/16 relative to 2014/15, down on the 8.8% increases seen in 2012/13 to 2013/14.
Overall, the renewable energy sector grew 3.5% in 2015/16, but the number of companies operating in the sector fell 5%, due largely to sharp contraction in the solar PV market.
Employment in solar PV fell from 16,880 in 2014/15 to 13,680 in 2015/16, with turnover falling from £2.5bn to £2bn.
The report also warns that the full brunt of policy rollback has yet to unfold through 2016/17.
This will feed through from changes to feed-in tariffs, closure of the Renewables Obligation to onshore wind and solar, reform of the Renewable Heat Incentive, loss of exemption from the Climate Change Levy and cuts to benefits for embedded generators.
Even so, the £14.7bn sector remains a major driver of UK green economy employment and has increased by an average of 6% since 2012/13.
The total number of jobs reached 129,940 by 2015/16, the REA report said. More than 24% of power, 6.2% of total heat supplies and 2.9% of transport fuel came from renewable sources in 2016, it added.
More than 16,000 jobs have also been added as a result of growth in electric vehicles and energy storage.
With less policy uncertainty and greater support, the sector has the potential to grow far faster than it has, the REA said.
It follows similar conclusions to those published in the Energy Institute's latest Energy Barometer survey in June, highlighting political risk to the low carbon transition.
But with government recognition of the importance of smart energy, energy storage and electric vehicles in its industrial strategy and anticipated new renewables support policies in its long-delayed clean growth plan, the long-term outlook for growth and green jobs could yet receive a boost.
REA chief executive Nina Skorupska: "It shows real progress that there were nearly 126,000 jobs in renewable energy in 2015/16. This is in addition to over 16,000 in energy storage and in electric vehicles, which is the first year we have reported on these sectors.
"What is deeply frustrating is that this growth could have been greater. Policy instability in Westminster has slowed growth. Our member companies are helping build a system that is reliable, low-carbon and more affordable than the previous one."