The fundamental energy unit in the metric system is the joule; 1 joule is defined as a force of 1 newton* applied over a distance of 1 meter. To work with large quantities, such as the amount of energy used in the United States in a given year, we use the unit exajoule, which is equivalent to 1018 joules, roughly equivalent to 1 quadrillion, or 1015, referred to as a quad.


To put these big numbers in perspective, the United States today consumes approximately 100 exajoules of energy per year, and world consumption is about 425 exajoules annually. In many instances, we are particularly interested in the rate of energy use, or power, which is energy divided by time. In the metric system, power may be expressed as joules per second, or watts; 1 joule per second is equal to 1 watt.


When larger power units are required, we can use multipliers, such as kilo-(thousand), mega- (million), and giga-(billion). For example, a modern nuclear power plant’s electricity production rate is 1,000 megawatts or 1 gigawatt. Levin’s points out that people are not particularly interested in having a certain amount of oil, gas, or electricity delivered to their homes; they are interested in having comfortable homes, adequate lighting, food on the table, and energy for transportation.