Keep economic systems going worldwide without creating inflation and putting our descendants into debt for generations to come.  renew-the-earth.org has been working on a calculation called the Human Renewable Energy Measure, HREM.

This measure is the standard of value for what we call Energy Currency.  The EC exchange functions within existing economic systems and parallels but does not eliminate traditional currency. 

EC can only be used for human survival needs; food and shelter and social necessities like health care, education and infrastructure.  When it enters a bank it is identified as Energy Currency and is destroyed so that it will not create inflation.  Similar, in this way, to the Green Back Dollar.