Energy is defined as the ability to do work, or to change the states and configurations of matter. Energy, a scalar quantity commonly measured in joules, watt-hours, or eqivalent tons of oil, TNT, etc., is conserved in systems, but may be transformed between different types, such as electromagnetic, electrical, thermal, and chemical. Renewable energy is energy from a source that is continually replenished.
By energy economy we mean the portion of the economy directly bearing on the methods of producing, distributing, and converting energy between its different forms. By renewable energy economy we mean that portion of the energy economy that is based on renewable energy.
In today's energy economy, the production of energy is highly centralized, requiring a vast distribution infrastructure that divides broadly into two categories: fuels and electricity.
Both electricity and fuels may be generated from renewable energy sources and distributed though the existing infrastructure. However, many renewable energy technologies enable the generation of enery at its point of use, bypassing part or all of that infrastructure.