If promises matter, it's been a very good stretch for renewable power.

At the end of July, Hillary Clinton said that as president she would aim to get 33 percent of the U.S.'s electricity from renewable sources by 2027 -- up from about 13 percent today (half of it from hydro). A week later, the Barack Obama administration said its new power-plant rules would require renewables to make up 28 percent of power capacity by 2030.

The political and legal obstacles to those goals are obvious. But behind them are more interesting questions: Is it even possible for the U.S. to increase so quickly the share of power it gets from renewables -- mostly solar and wind, given the public animosity to nuclear power -- by the end of the next decade? If so, what will it cost? And who would pay?

One way to answer the first question is asking whether there's precedent for so rapid a shift. The answer is yes, but with caveats -- and those caveats suggest that the pace of change Clinton proposes could come at significant cost. Promising more renewable power, without considering the costs, only panders to those who already support that goal; it doesn't advance the debate over how to achieve it, or enhance the consensus needed to carry it out.

In 2014, six states got more than 20 percent of their electricity from renewable sources other than hydro power. For many of those states, the ramp-up was very fast. The share of power generated in Kansas from renewables went from 0.9 percent to 22 percent in 10 years; in South Dakota, renewables grew even faster, from 2 percent in 2008 to 25 percent six years later.

If the U.S. could generate that rate of growth nationwide, it could easily meet the targets set out by Clinton, and even surpass them. But there's a catch: Much of the new renewable capacity was in states with landscapes ideally suited to large wind farms, built to export clean power to other states. That doesn't mean the U.S. couldn't do the same, but without a comparable source of external demand (and financing), which it won't have, that won't be as easy.

A better way to test the viability of the goals Obama and Clinton have set out is to look at the record of other countries. How fast can a nation move toward renewable energy when the right political consensus is in place?

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Here the data seems encouraging: A handful of European countries got 25 percent or more of their power from wind and solar last year, and many achieved that with amazing speed. Belgium got 1 percent of its power from wind and solar in 2004, and 26 percent last year. Germany went from 14 percent renewables in 2004 to 42 percent in 2014 -- a shift led by solar, which jumped from 1 percent of its generating capacity in 2004 to 21 percent last year.

Those countries show that Obama and Clinton's goals are attainable. But they also show the cost. The burst of renewable power forced utilities to rely less on their plants fired by fossil fuels. As a result, those utilities "took huge write-offs on their coal and gas plants," said Monne Depraetere, a European power analyst for Bloomberg New Energy Finance.

Pushing harder on renewables will put similar pressure on U.S. utilities. Between now and 2027, total U.S. generating capacity will increase just 0.25 percent, according to projections from the U.S. Energy Information Administration. Unless that changes, getting to 33 percent renewables by 2027 means eliminating about 145 gigawatts of power produced from fossil fuels -- an amount equal to almost 60 percent of the coal-fired generation the EIA projects would otherwise still be operating in the absence of Obama's Clean Power Plan.

What's different is who bears those costs. Unlike in Europe, most states still have regulated electricity markets, where utility companies recover the expense of building and upgrading plants by adding those costs to customers' monthly energy bills.

If a plant is forced to close because of environmental regulations, customers would probably be forced to keep making those payments all the same, according to Steve Mitnick, a former energy adviser to the governor of New York and consultant at McKinsey.

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In most cases, the expense of constructing the plants has been paid off. But recent Environmental Protection Agency rules, including 2012 regulations limiting mercury and other emissions, have forced utilities to spend billions of dollars on new equipment to capture those pollutants -- costs that are usually recouped over 15 to 20 years. "Customers will pay that off at the same time they would have to pay for new plants," as well as new transmission lines and upgrades to the grid, Mitnick told me.

It's hard to estimate that cost for any one state, but the total amount could be considerable. Last year the International Energy Agency projected the energy industry would have to idle about 165 gigawatts of fossil-fuel generating capacity that has yet to be paid off, with unrecovered costs of $120 billion. That's what's needed to keep atmospheric carbon dioxide to 450 parts per million -- the level experts say is necessary to limit the long-term rise in global temperatures to 2 degrees Celsius.

Proponents of renewable power are understandably reluctant to discuss that cost.

Clinton's campaign didn't respond to my e-mails seeking comment, and the section of her website that discusses her renewable power proposals makes no mention of the dollars required or where they'll come from. And a spokeswoman for the EPA told me that whether and how to shut fossil-fuel power plants is effectively the states' problem.

None of this means the targets set by Obama and Clinton can't be met, or that their cost isn't worth paying. On the contrary, the economic toll of inaction could far exceed the bill for shutting down coal plants more quickly. The climate benefits of retiring coal plants faster are obvious. And if the price of solar power keeps falling, the resulting savings could make the other costs easier to bear, according to Rob Barnett, an energy analyst at Bloomberg Intelligence.

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But the debate over renewables, as with climate change more broadly, needs to move past whether policy has to change (it does) and toward how much we're willing to pay for it -- and who gets the tab. Leaving that discussion for later, or pretending it doesn't need to happen, is the wrong way to turn promises into something real.

Christopher Flavelle writes editorials on health care, economics and taxation for Bloomberg View.