Generic "energy" is not fungible. The use-value, storability, transportability, and exchange-value of a gigajoule differ drastically depending on whether it's a

Generic "energy" is not fungible. The use-value, storability, transportability, and exchange-value of a gigajoule differ drastically depending on whether it's a gigajoule of enriched uranium, diesel fuel, electricity, sawdust, hot water, methane... An "energy-based currency system" is either a barter system that barters various energy-related commodities, or it's just as arbitrary as the USD, with the arbitrariness being set by whoever dictates standard conversion rates between different forms of actual and potential energy.