The UK-based wind sector all-rounder said the change to onshore wind support schemes in the UK was a key factor in the change.
"RES is not moving away from onshore wind in the UK, it is responding to the changes in the UK market," a company spokesperson told Windpower Monthly.
"RES will continue to have significant onshore wind operations in its other markets, internationally. RES will also continue to provide a range of offshore services to companies that operate in that sector along with an increasing focus on managing offshore assets for our clients," they added.
In 2015, a newly elected UK government closed the outgoing renewable obligation certificate system a year earlier than expected, except for a small number of projects that qualified for the grace period.
Onshore wind has also been excluded from the next contracts for difference (CfD) auction – expected by the end of 2016 or early 2017 – as only less-established technologies, such as offshore wind, will be able compete for support.
Access for onshore wind to the future auctions is yet to be clarified.
"In the UK, the closure of the renewable obligation, together with uncertainty about onshore wind's access to the contracts for difference regime, has resulted in an inevitable period of reduced activity in the UK onshore wind sector. These changes have led us to review the focus of our markets, activities and investment," RES Group chief executive Ian Mays said.
RES will reduce its UK workforce, with the onshore wind development team cut in size, the company said.
"There will be some reductions and enhancements internationally as the company adapts its focus to activities with the greatest potential to maintain sustainable growth and to ensure continued progress towards grid parity," the company said in a statement.
"Renewable energy markets around the world continue to change in response to economic and political priorities and in pursuit of parity with fossil fuel sources," Mays added.
The firm said it would place a greater focus on improving storage technology in the US, the UK and introducing it in other markets.
"With our total storage portfolio expanding, this is seen as an important driver for the business," the spokesperson said, adding asset management and solar were also "creating new opportunities".