Hillary Clinton is calling for a 700% increase in solar power. Is that realistic?

In the coming months, Hillary Clinton's campaign is planning to release a series of proposals for dealing with global warming. Her first installment came out Sunday and sets goals for major increases in US renewable power.

Specifically, she's vowing to boost the amount of wind, solar, and other renewables so that they provide 33 percent of America's electricity by 2027 — enough to power every home in the country:

Let's put this in perspective. Renewable energy currently provides just 13 percent of America's electricity, with hydropower providing 6 percent, wind power providing 4.4 percent, and the remainder coming from biomass, geothermal, and solar (down at roughly 0.5 percent).

If no new policies are put in place, official forecasts project that fraction would rise to 16 percent by 2027. If we add in President Obama's forthcoming Clean Power Plan — an EPA program to reduce carbon-dioxide emissions from power plants — then renewables are projected to grow faster, to 25 percent of US electricity by 2027.

Clinton wants to set that target higher still, at 33 percent. It's an ambitious goal. One key question is how she'd do it.

Clinton's goal calls for a 700% increase in solar power by 2020

Part of her plan involves accelerating the fast growth of solar. Clinton calls for US installed solar capacity to rise from roughly 20 gigawatts today to 140 gigawatts by the end of her first term — a sevenfold increase:

(Hillary Clinton campaign)

Is that plausible? US solar capacity grew 418 percent between 2010 and 2014 (it was starting from a small base). So a 700 percent rise between 2014 and 2020 is at least within the realm of possibility.

But it's undeniably a difficult task. The United States installed about 6.2 gigawatts of solar in 2014. Clinton is essentially vowing to up that rate to around 30 gigawatts per year during her tenure in office. Doing so would likely require solar to keep getting cheaper. It also might entail big policy changes — especially since a key federal tax credit for solar will expire in 2016.

Clinton's campaign says they'll release more specifics in the months ahead. For now, her campaign has put out a four-page fact sheet hinting at a few broad steps:

1) First, Clinton would veto any attempts by Republicans in Congress to scrap Obama's Clean Power Plan. This is mostly a defensive maneuver.

2) Next, she'd push to extend those federal tax credits for wind and solar that are slated to expire in the coming years. (The 30 percent tax credit for rooftop solar will shrink dramatically in 2016.) This part seems crucial, but it would also depend entirely on Congress. It's not something Clinton could do by herself. And note that many lawmakers would prefer to let these tax credits sunset, so this step is hardly guaranteed.

3) Clinton also wants to set up a "Clean Energy Challenge" that would give states and communities incentives to go even further than EPA's carbon standards. The campaign says it will offer more details on this in the coming months. One component? A "Solar X-prize" to reward communities that figure out how to speed up solar installation times. (One possible model here might be Bernie Sanders' "10 Million Solar Roofs Act.")

4) Clinton also has a smattering of proposals to boost public investment in clean energy R&D, expand transmission lines, and accelerate clean-energy deployment on public lands.

5) Finally, Clinton calls for assistance to coal communities that will inevitably suffer if coal keeps declining in favor of cleaner alternatives. No details on this yet, though her campaign says the government should "provide economic opportunities for those that kept the lights on and factories running for more than a century." (Note that the Obama administration has proposed a $3 billion aid package for struggling coal areas, but Congress hasn't shown much interest.)

It's not clear these policies alone would get the US to 33 percent renewables. We'll need a lot more detail here.

How ambitious is Clinton's renewables goal? It depends how you look at it.

On one level, yes, this is an ambitious target. If the US actually managed to get 33 percent of its electricity from renewables by 2027 and we kept most of our nuclear plants running (which currently supply 19 percent power), then the United States would be getting roughly half its electricity from zero-carbon sources.

That would be a huge departure from where we are today. It's also in line with the "50 percent clean energy by 2030" goal that green activist and billionaire Tom Steyer is demanding all candidates rally around.

On the flip side, Clinton's renewable goals aren't, by themselves, a comprehensive climate plan. Electricity only accounts for 38 percent of US carbon-dioxide emissions. Other major sources include transportation (i.e., cars, trucks, and planes that burn oil), industrial processes (i.e., cement plants or chemical plants that use coal or gas), homes and buildings that use natural gas for heating, and so on:

(Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2012)

On top of that, there's methane from landfills, oil and gas wells, coal mines, and livestock, plus other greenhouse gases from agriculture. Boosting renewable electricity is only one part of dealing with global warming. We'll see what else Clinton ends up proposing.

Could the US electric grid handle this much wind and solar?

Another question that's likely to come up is whether Clinton's vision for renewables is practical and cost-effective. As wind and solar grow, US grid operators will face challenges juggling these intermittent sources of power. After all, the sun isn't always shining and the wind isn't always blowing. And economic considerations do start creeping in.

Getting to 33 percent renewable power is certainly doable in principle, but the precise details will matter a great deal. How well the US grid copes will depend on the precise mix of renewables, on where the growth in wind and solar actually occurs, on whether utility regulators make certain policy changes, on whether storage becomes cheaper, and so on.

For broader context on these questions, my colleague David Roberts has written an excellent series on the challenges posed by fast-growing wind and solar. Part one looks at the difficulties of dealing with intermittent renewables. Part two looks at how solar and wind power face sharp economic limits once they expand past a certain point. Part three looks at potential utility reforms that might ease these constraints. Definitely read that series if you haven't already.

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Further reading: By the way, Martin O'Malley, one of Clinton's rivals in the Democratic primary, is arguably dreaming even bigger — calling for 100 percent renewable electricity by 2050. So there's some competition brewing around long-range goals. But, as always, the details and specifics are where the action is.

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