Energy is the capacity of a physical system to do work. The common symbol for energy is the uppercase letter E. The standard unit is the joule, symbolized by J. One joule (1 J) is the energy resulting from the equivalent of one newton (1 N) of force acting over one meter (1 m) of displacement. There are two main forms of energy, called potential energy and kinetic energy.

Potential energy, sometimes symbolized U, is energy stored in a system. A stationary object in a gravitational field, or a stationary charged particle in an electric field, has potential energy.

Kinetic energy is observable as motion of an object, particle, or set of particles. Examples include the falling of an object in a gravitational field, the motion of a charged particle in an electric field, and the rapid motion of atoms or molecules when an object is at a temperature above zero Kelvin.

Matter is equivalent to energy in the sense that the two are related by the Einstein equation:

E = mc2

where E is the energy in joules, m is the mass in kilograms, and c is the speed of light, equal to approximately 2.99792 x 108 meters per second.

In electrical circuits, energy is a measure of power expended over time. In this sense, one joule (1 J) is equivalent to one watt (1 W) dissipated or radiated for one second (1 s). A common unit of energy in electric utilities is the kilowatt-hour (kWh), which is the equivalent of one kilowatt (kW) dissipated or expended for one hour (1 h). Because 1 kW = 1000 W and 1 h = 3600 s, 1 kWh = 3.6 x 106 J.

Heat energy is occasionally specified in British thermal units (Btu) by nonscientists, where 1 Btu is approximately equal to 1055 J. The heating or cooling capability of a climate-control system may be quoted in Btu, but this is technically a misuse of the term. In this sense, the system manufacturer or vendor is actually referring to Btu per hour (Btu/h), a measure of heating or cooling power.

### Which Energy Source is Best?

It depends. Many alternative sources of energy are still being researched and tested. Technologies are continually being developed and enhanced to improve energy sources. Not all energies are ready for mass consumption, so you have to ask the right questions to find out which energy source does the job.

• Is it a renewable or nonrenewable source?
• What are the capital and setup costs?
• What are the ongoing operating costs?
• What size of energy storage is required?
• How efficient is it to produce one unit of energy?
• Can it be produced on a large scale?
• What is the cost to the consumer?
• What impact will it have on the environment?

Energy is lost to the environment during any energy transformation, usually as heat. Notice the heat from your computer or car after it has been in use for a while. Nothing is completely energy efficient.

### What are the Sources of Energy?

Primary energy sources (meaning energy is created directly from the actual resource) can be classified in two groups: nonrenewable or renewable. Secondary sources are derived from primary sources.

Non-Renewable Energy Sources – Energy from the ground that has limited supplies, either in the form of gas, liquid or solid, are called nonrenewable resources. They cannot be replenished, or made again, in a short period of time. Examples include: oil (petroleum), natural gas, coal and uranium (nuclear). Oil, natural gas and coal are called “fossil fuels” because they have been formed from the organic remains of prehistoric plants and animals.

Renewable Energy Sources – Energy that comes from a source that’s constantly renewed, such as the sun and wind, can be replenished naturally in a short period of time. Because of this we do not have to worry about them running out. Examples include: solar, wind, biomass and hydropower. Currently, about 20% of the world’s electricity comes from renewable resources. There is a global debate as to whether geothermal energy is renewable or nonrenewable.

Secondary Energy Sources –  Energy that is converted from primary sources are secondary sources of energy. Secondary sources of energy are used to store, move, and deliver energy in an easily usable form. Examples include electricity and hydrogen.

• Coal
• Wind Energy
• Uranium
• Petroleum
• Hydropower
• Solar Energy
• Geothermal
• Hydrogen
• Electricity
• Natural Gas
• Biomass