Are Clinton's solar goals feasible?
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Presumptive Democratic nominee Hillary ClintonHillary Diane Rodham ClintonIntel Dem decries White House 'gag order' after Bannon testimony 'Total free-for-all' as Bannon clashes with Intel members Mellman: On Political Authenticity (Part 2) MORE has pledged to reach 140 gigawatts (GW) of solar by 2020, the centerpiece of her Vision for Renewable Power. Just how ambitious is that goal, and is it feasible?

The Vision would require quintupling solar photovoltaic (PV) capacity during her first term as president. Solar capacity more than quintupled from 2010 to 2014, so the growth rate is not unprecedented.

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However, percentage growth becomes more challenging as the base grows. Clinton's 140 GW target dwarfs the 54 GW of solar PV forecast by the Energy Information Administration (EIA) for 2020 under current policies. Her target even exceeds the EIA's outlook for solar in 2030, thus speeding up deployment by over a decade.

Past EIA outlooks have tended to underestimate solar growth. However, more optimistic projections by groups such as Green Tech Media predict far less solar power by 2020 than Clinton targets.

How would Clinton's target impact climate-warming emissions? The answer will depend on where the solar panels are sited, the performance they achieve and the electricity sources they replaces. Rough calculations based on EIA data for solar output and average electricity emission rates suggest that the additional 86 GW of solar would save roughly 300 million metric tons of carbon dioxide annually. That's about 6 percent of U.S. emissions from all sectors.

Beyond the solar target, Clinton pledges that within a decade of her inauguration, the U.S. will be generating enough "clean renewable energy" to power every home. The EIA forecasts that residential electricity consumption will reach 1400 billion kWh by 2027, with renewable energy generating 1,040 billion kWh if the Clean Power Plan is implemented. Clinton's goal for solar by 2020 would close about half of that gap, leaving seven years for more solar plus other sources like wind to reach this target. In other words, achieving the 2020 solar target would likely put the 2027 renewable energy goal within reach.

Clinton's Vision for Renewable Power is clearly ambitious. Does she have a viable plan to attain it?

As is typical of campaign proposals, the details of Clinton's plan remain vague. Clinton pledges to implement the Clean Power Plan, though the EIA forecasts this will have little influence on solar until later in the 2020s.

When the Vision was issued last year, it proposed to extend renewable energy tax credits that were set to expire. However, the credits were already extended by Congress last December, and the EIA's forecast accounts for that extension.

Thus, achieving the solar target will depend on a series of initiatives that are only vaguely described in the Vision fact sheet. These include competitive grants to states; a Solar X-Prize for communities that foster rooftop solar; deployment of solar on public lands and federal buildings; and increased investment in clean energy research and development, including the Department of Energy's ARPA-E (Advanced Research Projects Agency-Energy) program. The scope and cost of these initiatives remain to be determined.

Success can breed success. Costs for renewable energy tend to drop as more capacity is installed. This reflects not only economies of scale but also advances in technologies to meet growing demand. A recent literature review found that solar costs drop roughly 23 percent for each doubling of capacity, though estimates of this "learning rate" vary widely. How quickly solar costs continue to drop will be crucial to whether Clinton's Vision is both achievable and affordable.

In sum, Clinton has issued an ambitious, but not implausible, Vision for Renewable Power. Achieving its solar target will depend on the details of the policies, and whether they can accelerate the ongoing decline in solar prices.

Cohan is associate professor of civil and environmental engineering at Rice University.