Energy and Power Units: The Basics

Over the centuries, scientists followed diverse paths as they investigated energy and power. So each type of energy - electrical, mechanical, chemical, thermal and nuclear - acquired its own system of measurement, and each energy-related industry developed its own terminology. Car dealers talk about horsepower. HVAC contractors install tons and BTUs. Electric utilities deliver kilowatt-hours. Scientists refer to newtons and joules.

In 1960, the International System of Units (SI) was derived from the metric system to provide a standard vocabulary for all things physical. The SI is built on seven base units (see Table 1) from which all other physical quantities can be derived. Table 2 lists some standard derived units. For example, a newton (derived unit of force) is defined as one kilogram (base unit of mass) accelerated at the rate of one meter (base unit of length) per second (base unit of time) squared. Table 3 lists some standard prefixes used to denote multiples and fractions of units. For example, a megawatt (MW) is equal to a million (10⁶) watts; a milliwatt (mW) is one-thousandth (10^-3) of a watt.

Energy-related industries are beginning to use SI terminology, but traditional terms still dominate. Many car companies now list engine power in kilowatts, but in parentheses after the horsepower rating: 187 hp (140 kW). Starting with a brief tutorial on energy, power and force, the next few sections review the units used most commonly in renewable energy applications.