“If the EROI [“energy return on investment”] on an oil well is 20:1, you get twenty units of energy out for every unit you put in. Twenty to one is pretty good—a lot better than, say, using Canadian tar sands and melting them down to get usable oil. That might produce an EROI of 5.2:1 by some recent estimates. Corn ethanol for oil? Once you’ve figured in all the energy it takes to grow the stuff and process it, you’re lucky to break even. Charles Hall, a professor at the State University of New York, argued recently that “to offer any remotely viable contribution to society, a liquid fuel should not be dependent on subsidies from petroleum and should have an EROI of at least 5:1.” Solar panels: somewhere between 2.5 and 4.3:1, at least for now.”
McKibben - Eaarth: Making a Life on a Tough New Planet, p. 31.
It’s nice to see the numbers on this, but it’s worth noting that a German/Australian research team has managed to bring the efficiency of solar cells to 40%, while current panels have about 12-17% efficiency. The article doesn’t translate this into EROI, but it’s heartening nonetheless.