Montana-Dakota Utilities Co. and Otter Tail Power Co. estimate the current rules could drive up their customers' monthly bills by 18 percent to 30 percent annually. If the problem is not remedied, it could undermine public backing for wind power development in North Dakota, Public Service Commissioner Tony Clark said. The state has more than 700 megawatts of wind generation capacity, with more than 3,000 megawatts of projects planned.