“Today I challenge our nation to commit to producing 100 percent of our electricity from renewable energy and truly clean carbon-free sources within 10 years.”
-Al Gore, 2008
Our political leaders have a huge influence in how our population views technologies. The former statement, while laudable, is distilled to the point of necessitating further analysis. While the goal of ubiquitous carbon-free energy sources is certainly worth pursuing, it is vital that we do not discount the carbon footprint entailed in the adoption of those sources.
Let’s just take a look at solar photovoltaic (PV) systems. According to  and , the energy required to make a solar panel (by Siemens-like processes in 2007) was 4354 MJ/m2, which equates to 1210 kWh/m2 of panel produced. If this panel produces about 168 kWh/m2 in a year, then the estimated energy payback time (EPBT*) is about 2.2 years. This is great news. However, if you then consider the carbon footprint of creating this solar panel and divide it over the panel’s lifetime (~30 years), the aggregate carbon emissions amount to 32 g/kWh produced. Fortunately, given improvements in technology, this value was expected to drop to 24 g/kWh by the end of 2011. So even though solar panels do not release CO2 during their normal operation (because they don’t need fuel to operate), the manufacture of solar panels does have a nontrivial carbon footprint. The upside of the story is that this footprint is still dwarfed by those of coal, oil, and natural gas (see table III below).
So far, I’ve spoken only of the carbon emissions from the manufacture of PV arrays. This is only one side of the CO2 payback time equation. Another vital consideration is the carbon that is displaced by using solar energy in lieu of dirtier energy sources from the existing energy infrastructure — namely, coal and natural gas. To quantify this carbon offset, one must consider both the energy source that would have been used were it not for the solar panel, as well as the amount of power being displaced by the solar panel. Furthermore, the amount of power provided by a panel is a function of both its efficiency (largely driven by technology) as well as the solar panel’s environment. For example, according to , it would take twice as much time for a panel in the UK than in California to offset the energy used for its production. This is due to California’s favorable sunshine conditions of about 1,700kWh/m2 per year, compared to the UK’s less favorable 700-900 KWh/m2 of solar energy per year.
So, in the end, solar power is a lot cheaper in terms of CO2 emissions and this is why I support their adoption. But it is at times appropriate to recall the famous quip from science fiction author Robert Heinlein: “there ain’t no such thing as a free lunch.”
*“EPBT is defined as the number of years a PV system must operate before it generates sufficient energy to equal the amount it consumed in manufacturing” 
 P. Zhai and E.D. Williams, “Dynamic Hybrid Life Cycle Assessment of Energy and Carbon of Multicrystalline Silicon Photovoltaic Systems,” accepted for publication by Environmental Science & Technology (Sept.3, 2010).
 Y. Jiao, A. Salce, W. Ben, F. Jiang, X. Ji, E. Morey, and D. Lynch, “Siemens and Siemens-like Processes for Producing Photovoltaics: Energy Payback Time and Lifetime Carbon Emissions” JOM, 63 (1) (2011), pp. 28–31. Can be accessed here: http://www.springerlink.com/content/93h4wh6718251270/fulltext.pdf