2.1 Units of energy and power
A host of different units are used to describe energy and power, which can seem a bit confusing. But the following Table 1, where we have listed and described some of the most frequently used units, should clarify the meaning of the various terms.
|Joule (J)||Main scientific unit of energy||Watt (W)||Main scientific unit of power – defined as 1 joule per second|
|Kilojoule (KJ)||Equal to 1000 (103) joules||Milliwatt (mW)||Equal to 1000th of a watt (10-3)|
|Megajoule (MJ)||Equal to 1 million (106) joules||Kilowatt (kW)||Equal to 1000 (103) watts|
|Gigajoule (GJ)||Equal to 1 billion (109) joule||Megawatt (MW)||Equal to 1 million (106) watts|
|Exajoule (EJ)||Equal to 1 quintillion (1018) joules||Gigawatt (GW)||Equal to 1 billion (109) watts|
|Kilowatt-hour (kWh)||The amount of energy produced by a power of 1 Kilowatt (1 kW) in one hour|
|Megawatt-hour (MWh)||The amount of energy produced by a power of 1 Megawatt (1 MW) in one hour|
|Gigawatt-hour (GWh)||The amount of energy produced by a power of 1 Gigawatt (1 GW) in one hour|
As you can see from Table 1, most of the terms are used to describe larger quantities of the basic units – joules or watts.