Permission should not have been allowed to site two wind turbines that are larger than Big Ben on a mountain above their homes, according to opponents.
They are concerned the 125m (410ft) turbines will have an impact on wildlife and cause noise pollution at Ynyshir, near Porth, Rhondda Cynon Taf.
One resident, who lives nearby, said he was concerned about natural drainage and surface water run-off at the site.
The developer was not available for comment.
Resident Philip Thomas, who lives beneath where the turbines are to be located, said: “My major concern is natural drainage and surface water.
“This area was under water in February during storm Dennis, and has a history of landslides and flooding.”
He claimed that people living in the locality were not given adequate notice of the proposed development in order to voice their objections, and that he only found out about it recently by chance.
However, the council said it had held a “consultation process undertaken in accordance with relevant procedures set out in Welsh Government legislation” in 2017.
Opponents have since set up a Facebook group with more than 170 members.
Cenin Renewables, which was granted planning permission for the development in 2017, was not available for comment.
But its website said “generating local income from the wind turbines means we can support the local community” and the the two turbines, which would each produce 2.5MW of electricity, “would supply the total energy needs of approximately 4,500 houses for the next 20 years”.
A Rhondda Cynon Taf council spokesperson said a planning application for two wind turbines at Llwyncelyn Farm was granted consent by the planning and development committee in November 2017.
“There is no requirement in the legislation to write to every household in the area, as is the case with all applications of this scale,” they said.
“However, the closest surrounding properties were notified directly, while notices were published both on site and in the local press.”
They said 34 objections were received “and these were shared with the committee to consider in determining the application”.
“The potential impacts upon the amenity of residents and site drainage were also fully-considered, as set out in an officer report to the committee and the minutes of its meeting in November 2017.”