(Watt (symbol: W) is the SI unit of capacity (energy per unit of time). This unit is named after the Scottish engineer James Watt. A light bulb with a capacity of 60 Watt uses 60 Joule worth of energy per second. A more ancient unit of capacity is horse power (hp), which is more or less equal to the following calculation: 1 pk = 735,498 W, whilst 1 hp is approximately equal to 746 W.
A practical, non-SI unit which is derived from the watt directly, is the kilowatt-hour (kWh): 1 kWh = 3,6 MJ (mega joule). The multiplication of the watt (joule per unit of time) with a unit of time causes a certain unit of energy. To calculate how much a device of, for example, 10 watt, consumes in 24 hours, you use: 10W*24h=240Wh=0,24kWh
Please note: a lot of devices show what the device can use maximally, although that does not mean the device uses this amount of energy continuously.
A megawatt (symbol: MW) is equal to a million (106) watt. De production capacity of power plants is often expressed in megawatt, also referred to as MWe (in which the 'e' stands for electric). The thermal capacity (the amount of heat created) is always higher and gets referred to as MWth. The created heat will partly be wasted when it is turned into electricity.
The absorbed capacity of devices is usually expressed in volt-ampere, as opposed to the delivered capacity, which is expressed in watt. An exception to this rule is a light, because people are more familiar with the amount of light (delivered capacity) a light bulb produces for a certain absorbed capacity, expressed in watt. Volt-ampere shows the observable capacity (tension times current), watt shows the active part (the part where tension and current are 'in phase'). Because tension and current are continuously in phase with one another when they are in the same current (in other words, the direction of the current is always the same in regard to the tension), the visible capacity will be equal to the active capacity. LED lighting does not need a lot of Watt in order to get to a high light performance, which makes these lights way more energy friendly than the regular lights we may use.
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