HELENA, Mont. -

An economist for the Montana Consumer Counsel says NorthWestern Energy's proposed purchase of 11 hydroelectric dams could cost ratepayers $800 million over 15 years unless more risk is shifted to the company.

Economist John Wilson questioned some of the assumptions behind company's argument that the deal helps consumers.

Among the assumptions that Wilson questioned was that a future carbon tax will make other forms of power generation more expensive.

The company said Friday it stands by its analysis.

NorthWestern Energy has asked state regulators to approve the $900 million purchase of the dams from PPL Montana.

The Montana Consumer Counsel is a state agency that represents consumers in utility hearings, and the counsel asked Wilson to analyze the dam proposal.