MARS HILL – The wind turbines atop Mars Hill Mountain are spinning to life – for brief spurts, at least – as testing gets under way at the wind farm, which is expected to begin generating energy by the end of the year.

With 20 turbines planted along the mountain’s ridge and eight left to assemble, the Mars Hill Wind Farm is the biggest wind power operation to come to New England.

Evergreen Wind Power LLC of Bangor has worked for four years and is spending about $55 million to develop the wind farm. It is expected to generate about 42 megawatts of electricity, or enough to supply about 45,000 Maine homes at full capacity. Whether the electricity will be sold to customers in Maine or elsewhere is still being negotiated.

Project manager Andy Perkins said Thursday that while some of the 50 or so workers at the site are installing the last turbines on the north end of the mountain – they will be placed at a lower elevation than the turbines on the ridge – other crews are conducting “mechanical completions.”

For each turbine, workers are running through a comprehensive checklist that includes inspecting all internal and external parts, torquing the bolts, making sure all cables are tight, and ensuring that the turbine alignment is correct. Perkins said the time it takes to finish a check varies from turbine to turbine.

“We’ve done mechanical completions in a day, and we’ve done them in a week,” Perkins said. “It depends on how long the punch list is and how much detail we have to go into.”

Crews have mechanical completions done on about nine of the turbines. Once the wind towers have been checked out, Perkins said the next phase is to prepare them for commission. That includes going inside each tower, setting all the computers up, and going through the electronics and programming.

As workers complete these last steps, Perkins said that people eyeing the mountain will see the turbines spinning once in a while. Crews will turn them on and off as the commissioning proceeds.

“We still have a lot of work to do, but we’re making progress,” Perkins said. “We’re very hopeful that we’re going to be spinning by the end of the year.”