Examines the importance of energy return on investment (EROI) as a useful metric for assessing long-term viability of energy-dependent systems. Here, focuses on the methods, applications, and analyses for determining EROI for solar power and solar energy technologies.

Photovoltaic (PV) electricity generation has grown to about 17 GW in the United States, corresponding to one tenth of the global capacity. Most deployment in the country has happened during the last 6 years. Reflecting back in time, in early 2008 this author and his collaborators James Mason and Ken Zweibel, published in Scientific American and in Energy Policy a Solar Grand Plan demonstrating the feasibility of renewable energy in providing 69% of the United States electricity demand by 2050, while reducing CO2emissions by 60% from 2005 levels; the PV contribution to this plan was assessed to be 250 GW by 2030 and 2900 GW by 2050 [1]. The DOE's more detailed SunShot vision study, released in 2012, showed the possibility of having 300 GW of PV installed in the United States by 2030, and 630 GW by 2050. Assessing the sustainability of such rapid growth of photovoltaics necessitates undertaking a careful analysis because PV markets largely are enabled by its promise to produce reliable electricity with minimum environmental burdens. Measurable aspects of sustainability include cost, resource availability, and environmental impact. The question of cost concerns the affordability of solar energy compared to other energy sources throughout the world. Environmental impacts include local-, regional-, and global-effects, as well as the usage of land and water, which must be considered in a comparable context over a long time, multigenerational horizon. Finally, the availability of material resources matters to current and future-generations under the constraint of affordability.