The IEEE Spectrum magazine has a preview of a book looking at the EROI (energy return on investment) for solar PV, based on 3.5GW of actually installed capacity in Spain. The authors suggest that based on their case study, the EROI for utility-scale PV, when all the associated energy expenditures are accounted for, is substantially lower than the value of ~7 which is commonly cited. It’s worth noting, however, that EROI is not something being optimized for right now. We’re very much focused on the plain old ROI, and in a world without a meaningful carbon tax (and, indeed, many subsidies for fossil fuels) even if you’re building renewable energy installations, you’re going to tend to use the cheapest energy available in that pursuit. The article also points to another study, suggesting that an EROI of 12 or so is necessary to support “modern society”… but that has to depend pretty intimately on how efficiently you utilize your energy, and what you think constitutes “modern society”. Either way, the EROI for fossil fuels is steadily declining as we pursue more and more “unconventional” reserves, so we’ll have to come up with a new solution, whether we want to or not.