It powers our cars and appliances, heats our homes, and lights our workplaces. The science of thermodynamics teaches that energy can take on many different forms, each equivalent. Energy can be stored in a compressed spring, a rotating shaft, a pressurized vessel, a magnetic field, etc.
Heat is a form of energy.
Equivalency implies that each type of energy can be measured in the same units. For example, we could sell electricity in calories or measure food energy in kilowatt-hours. This would baffle consumers, of course, but would be perfectly correct thermodynamically. Table 1 shows the basic energy types and other characteristics associated with each renewable energy resource. The type of energy may restrict how the resource can be used or at least imply that some uses may be more economical than others.