What’s the real cost of energy from coal plants?

State energy regulators did the right thing this month when they asked Puget Sound Energy to do a more rigorous assessment of the utility’s continued use of coal-fired electricity from four coal plants in Colstrip, Mont.

The Utilities and Transportation Commission said it needs more information to determine whether the state’s largest investor-owned utility should or shouldn’t include the coal plants in its 20-year energy portfolio.

The UTC noted, among other things, that the utility failed to consider the possibility of a carbon tax or tougher federal environmental regulations on coal plants, which could drive up the cost of coal power.

This state has gone to great lengths to eliminate coal power production within its boundaries with the phased shutdown of the Centralia coal plant. Yet PSE relies on coal power from the four units in Montana and other coal power sources to supply about 30 percent of the electricity consumed by its 1.1 million electricity customers, including customers in Thurston County.

Weaning Washington off coal is a key plank in a greenhouse gas emissions reduction strategy embraced by Gov. Jay Inslee and the Democratic members of his climate change workgroup. It is also a top priority of the environmental community. Climate scientists have made the case that greenhouse-gas emissions from coal-fired power plants are a major contributor to global warming.

When the UTC announcement came out requiring more PSE homework on future costs of coal, the Sierra Club hailed it as a major victory in its “Beyond Coal” campaign. That’s a bit of a stretch. At this point all the UTC has asked for is more information, more analysis, more number-crunching on the potential costs of keeping coal power in PSE’s energy portfolio.

It’s a reasonable request. The plants were built in 1975 and 1976 and face major upgrades in the years ahead to keep pace with federal requirements to reduce regional haze, mercury emissions and other air pollutants. The UTC estimated that Puget Sound Energy may have to invest anywhere from $69.7 million to $200 million by 2018 in the two oldest Colstrip plants.

Puget Sound Energy is in a curious position: Utility officials have said they want to continue to green up their energy portfolio and reduce reliance on coal power. At the same time, they’ve contended that the lowest-cost option for ratepayers is to continue to operate the Colstrip plants, calling them steady, reliable sources of power that save ratepayers hundreds of millions of dollars every year.

But what environmental price comes with those lower utility bills?

Evidence mounts that the true environmental costs of coal power are not reflected on utility bills, but rather in the air we breathe and the greenhouse gases coal power produces. That’s why the potential of a carbon tax can’t be ignored.

The UTC, and ultimately the ratepayers, need more information on what those future costs look like, with or without coal.