The owner of Montana's largest operating wind farm, near Judith Gap, has proposed adding 35 turbines, which would increase its power-production capacity nearly 40 percent, officials at NorthWestern Energy confirmed.

Invenergy, based in Chicago, has pitched the expansion to NorthWestern, the utility buying the electricity currently produced by the wind farm north of Harlowton in central Montana.

NorthWestern should decide soon whether it wants to buy power that would be produced by the additional turbines, said John Hines, chief supply officer for NorthWestern.

"We're evaluating their offer and looking at it (versus) other electricity portfolio alternatives," he said Thursday.

NorthWestern has 320,000 customers in Montana, and obtains most of its wholesale electricity on the open market or through long-term contracts with energy producers, such as Invenergy. That power is then sold to NorthWestern's residential and business customers across Montana.

Invenergy, which operates the 135-megawatt wind farm, did not return a message seeking comment.

Under its 20-year contract to sell power to NorthWestern, Invenergy must give NorthWestern the first shot at buying electricity from any expansion up to 188 megawatts.

If NorthWestern turns down the offer, Invenergy can sell the power somewhere else, but not for a lower price than offered to NorthWestern, Hines said.

Price details of the offer to NorthWestern were not made public.

However, Hines said Invenergy is proposing to add 52.5 megawatts of production to the wind farm, or 35 new turbines. Each turbine can produce up to 1.5 megawatts of power. The project currently has 90 turbines, each of which is a large tower with a single blade.

One megawatt of wind power provides enough electricity for 240 to 300 homes.

At 135 megawatts, Judith Gap is the only major wind-power project in Montana.

NorthWestern is paying about $30 per megawatt-hour for electricity purchased from the Judith Gap wind farm, which began operating in 2006. Other costs associated with the power increase the bill to about $38 per mwh, but those costs are projected to increase this year, NorthWestern officials have said.

The cost is considerably less than the $56 per mwh that NorthWestern is charging customers for its "portfolio" power, which is electricity from a variety of sources, purchased on the market.