An Energy is the power to do work. Making new items from recycled ones takes less Energy than making products from brand new materials.

Other /More definition:
An Energy is the capacity for doing work as measured by the capability of doing work (potential Energy) or the conversion of this capability to motion (kinetic Energy). Energy has several forms, some of which are easily convertible and can be changed to another form useful for work. Most of the world's convertible Energy comes from fossil fuels that are burned to produce heat that is then used as a transfer medium to mechanical or other means in order to accomplish tasks. In the United States, electrical Energy is often measured in kilowatt-hours (kWh), while heat Energy is often measured in British thermal units (Btu).

Other /More definition:
Energy is the capacity to do work by performing mechanical, physical, chemical, or electrical tasks or to cause a heat transfer between two objects at different temperatures.