The Campaign for the Protection of Rural Wales has claimed plans unveiled by Powys County Council to produce an additional 973 gigawatt hours by 2026 using wind and solar farms are deeply flawed.
But council chiefs have claimed the measures put forward in its Local Development Plan are necessary to meet the Government's request to use renewable energy sources.
Under the proposals, areas such as the Kerry Hills – which is part of Shropshire Hills Area of Outstanding Natural Beauty – and Cilfaesty, which is known for its population of red kites, could become sites for wind turbines or solar panels.
Details have been listed under the Further Focussed Changes section of the authority's plan, and it is hoped it could provide 106 per cent of Powys' electricity needs.
Currently, experts have said wind produces 312.7 megawatts in Powys. There are no major solar farms.
Peter Ogden, CPRW director, said: "If it is implemented, the policy provides an invitation to developers to cover almost 40 per cent of Powys with combinations of wind turbines or major solar arrays.
"The confirmation of this last minute and totally new proposed approach could precipitate a massive change in the character of the uplands of Powys.
"CPRW will continue to do everything they can to prevent this pernicious humiliation of the green heart of Wales."
Montgomeryshire MP Glyn Davies said: "My main concern is that we are in danger of opening a free-for-all for developers to move into areas where they couldn't previously and set up large wind and solar farms.
"I can't see Powys councillors agreeing to this – the Welsh Government is dictating to Powys exactly what it should do. I'm going to meet with the CPRW to discuss this."