BILLINGS (AP) - Fifty more turbines have gone online at the state's first large-scale wind farm near Judith Gap, and officials hope to have all 90 up and running by the end of January.

The Judith Gap Wind Energy Center began producing electricity in late November.

NorthWestern Energy is buying power from the 135-megawatt wind farm for customers in central and western Montana. Each turbine provides energy for about 350 to 400 homes _ or essentially 8 percent of NorthWestern's supply.

Before the Judith Gap wind farm came online, small wind farms and individual turbines dotted the Montana landscape but only produced about 1 megawatt of power. When all 90 turbines at Judith Gap are spinning, they will produce 135 times that amount.

The contract between NorthWestern and Invenergy, the Chicago-based company that owns the facility, permits up to 188 megawatts of power, leaving the door open for expansion.

John Bacon, onsite manager for Invenergy, said he expects to know by spring whether another 30 turbines will be added at the site.

The problem with wind energy can be its unpredictability.

Last week, the wind was barely breezing through the gap at 10 mph _ well below the optimal 20 mph. Just 50 miles away, it was gusting furiously in Big Timber.

"This should be producing 76 megawatts," Bacon said, "but we're only producing 24."

Every morning, Bacon calls NorthWestern to report how much energy he expects to send online. To make the best forecast, he frequently checks the National Oceanic and Atmospheric Administration Web site, although NorthWestern plans to add three meteorological towers at the site to improve his predictions.

Still, the fickle nature of wind requires energy suppliers to coordinate backup supplies, meaning "free" wind isn't exactly free. Besides the cost of developing the wind farm, the cost of wind power is blended with the cost of backup energy.

NorthWestern spokeswoman Claudia Rapkoch said it's hard to say whether wind energy will lower or increase energy costs. Beyond the possible 30-turbine expansion at Judith Gap, the company has no immediate plans to include more of Montana's wind in its energy portfolio, she said.

"This is what we feel is the right amount for the time being," Rapkoch said. "From a default supply standpoint, there's nothing immediately on the horizon."