Fossil fuels face resource depletion, supply security, and climate change problems; renewable energy
(RE) may offer the best prospects for their long-term replacement. However, RE sources differ in many
important ways from fossil fuels, particularly in that they are energy flows rather than stocks. The most
important RE sources, wind and solar energy, are also intermittent, necessitating major energy storage as
these sources increase their share of total energy supply. We show that estimates for the technical potential
of RE vary by two orders of magnitude, and argue that values at the lower end of the range must
be seriously considered, both because their energy return on energy invested falls, and environmental
costs rise, with cumulative output. Finally, most future RE output will be electric, necessitating radical
reconfiguration of existing grids to function with intermittent RE