Energy is defined as the amount of work that can be performed by a force. It exists in different forms and is each named after a related force.
Science says that there is no absolute measure of energy, because energy is defined as the work that one system does on another. Only the transition of a system from one state into another can be specified and thus measured.
Ways to measure energy
Energy can be measured as the work done: (Force x Distance = Work)
Based on the definition, the standard unit of energy measurement is a joule (J). Joule refers to the energy (or work) required to move an object with 1 Newton of force across 1 meter.
Energy can also be expressed in calories. A calorie (cal) is the amount of heat needed to raise a gram of pure water from 14.5°C to 15.5°C under standard atmospheric pressure. One calorie is equal to 4.1855 joules.
Energy can be measured in British thermal unit (BTU) as well. One BTU is the amount of energy required to raise the temperature of a pound of water by 1°F.
The most common unit of energy measurement is a watt. One watt is equivalent to one joule of work per second and measures the intensity of radiation over 1 square meter surface. The watt is a standard unit to express electrical power and capacity if electric generators.
Horsepower is another unit of measuring energy. 1 HP is equivalent to 745.7 watts.
Accordingly to SI, accepted units of measurement of energy is the joule, kilowatt hour (1kWh = 1000 watts) and the BTU. The last two are larger units in which energy can be expressed.