Across its eight-state system, Xcel predicts that well over half its electricity will come from renewable sources by the mid-2020s. It will be one of the cleanest large utility companies in the country.

Now, to be clear, the low bids that Xcel is getting include some federal subsidies for clean power. Those subsidies are entirely defensible, but both parties in Congress have agreed to phase them out in a couple of years. Mr. Fowke is jumping now in part to lock in the subsidized prices.

Yet costs for renewable technologies are coming down so much that by the time the federal subsidies expire, wind turbines and large-scale solar arrays will still be competitive in large parts of the country.

The same trend is occurring all over the world, even in countries that do not offer subsidies, with renewable projects routinely beating fossil-fuel projects in countries like Mexico and India. We are confident more price declines are coming.

The costs of huge batteries are also falling, and it looks as if they will turn out to be a big help in managing the variability of wind and solar power. Xcel is already testing a battery project near Denver, and it may buy more batteries as part of the new plan.

How, exactly, did the cleanest energy technologies get on a path to become the cheapest?

In a way, the story is as old as Henry Ford and his Model T, or in more recent times, the amazing progress of computer chips.