Energy, the ability of a system to do work, has a variety of measurement units, but the most basic unit of energy is the joule. Named after James Joule, the joule was developed to explain the relationship between heat and energy. A joule is roughly the amount of kinetic energy in a 1-kilogram object traveling at 1 meter per second.
Although the joule is the most basic unit of energy, it is too small to represent most things in everyday life or in a science lab. Other units for energy include the watt, as seen in light bulbs; the kilocalorie, as seen in food; and the BTU (British thermal unit) used for air conditioners or furnaces. A watt is the number of joules generated per second. A kilocalorie is 4,184 joules, the amount of energy that will heat 1 kilogram of water by 1 degree Celsius. Meanwhile, the British thermal unit is the amount needed to heat 1 pound of water by 1 degree Fahrenheit. Smaller units of energy exist, such as the electron volt, but they are not useful for everyday life. The electron volt is the energy gained by one proton or electron when passing through a 1-volt electrical field.