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Figure A2. Relative historic economic growth trends for GDP, Energy use and Economic
Efficiency ($/Wh) for purchased energy use [36]: Curves each indexed to 1971 GDP, with
Energy use and Economic Efficiency then scaled by the ratio of their own growth rate to
GDP growth rate. The relationship is lnGDP = lnEnergy + lnEE. Each curve is shown
with an exponential fit with indicated doubling rate (Table 4 in [29]).© 2011 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).