ST. PAUL, Minn. (AP) - An independent report on the proposed Big Stone II power plant says the utilities that want to build the coal-fired plant have underestimated their costs.

The report to the Minnesota Public Utilities Commission also says the utilities have overestimated the costs of energy alternatives.

It was prepared by Boston Pacific Co. at the PUC's request. The plant would be built in northeast South Dakota, but the utilities need the PUC's permission to run power lines into Minnesota.

"In general we believe the range … used in the applicants' analyses were not appropriate," the report said. "Put another way, they were out of line with current 'best practices.'"

The utilities, including Otter Tail Corp. of Fergus Falls, Minn., and Bismarck's Montana-Dakota Utilities, say they're not that far from the report's numbers on construction and fuel costs. And they challenge the report's assumption on the size of a future carbon tax.

Environmental groups say the report backs up their opposition to Big Stone.

"We're pleased that yet another set of independent eyes have looked at the record, and largely agreed with the points that we have been making all along," said Beth Goodpaster, who represents several of the groups.

PUC executive director Burl Haar says the utilities and others can respond to the report at a hearing Nov. 12, and the commission will take up the issue next in mid-January.

The utilities say the plant is needed to meet a growing demand for energy. Opponents are pushing for clean-energy alternatives because coal-fired plants produce pollutants such as carbon dioxide.

The Minnesota PUC ordered the report in June. It had been expected to make a decision about the permit after four years of debate but four commissioners appeared split, with the fifth expressing frustration about the two sides' conflicting cost projections.

Boston Pacific said the utilities' projected carbon tax costs started low and didn't factor in inflation. The consultant said construction cost projections were below Boston Pacific's own low-end figures and that any higher costs would fall on ratepayers.