# Power

## Power

The standard unit of Power is the Watt, symbol ‘W’ 1000 Watts = a kilowatt, symbol ‘kW’ In this context the number of kW assigned to a process or piece of equipment represents the amount of energy that that process or equipment consumes or produces per second.

This can also be explained using the ‘electricity is water analogy’. If someone is using a hosepipe to water their garden then every second a certain amount of water will be pushed out of the hose onto the garden. You might measure this in litres per second or gallons per minute or any ‘capacity per time’ kind of measure. Similarly a spring in someone’s garden might produce so many gallons of water per hour that flows into a small stream (nice garden!). In either case this ‘rate of water flow’ or ‘amount of water per second’ tells us something important about a source of water or something consuming water. A quantity of water flow under pressure can be considered the equivalent of ‘power’. In a similar way, electrical power may be thought of as the ‘volume’ of electrical energy used or produced by something in a second. Electrical power is in effect the electrical energy per second that might be thought of as ‘flowing’ into or from an electrical device. So a piece of equipment that has a power rating of 3 kiloWatt (3000 Watt), such as a kettle, requires a relatively high flow of electrical energy per second and so consumes a lot of electrical energy whilst it is switched on. On the other hand a device that consumes 10 Watts of power (for example a low power light bulb) requires a comparatively small flow of electrical energy per second.

Importantly, note that the pressure and rate of water consumption or production (the power) doesn’t tell us all we might need to know about the water being moved. For a start the actual amount of water used (measured in litres, or gallons, or cubic metres, etc) depends how long the flow continues for. So for example if someone filled a bucket for 10 seconds with a hose supplying half a litre per second they would have used 5 litres of water. If someone else had a tap dripping at a rate of a litre per hour (a much slower rate of consumption), for 12 hours they would use 12 litres of water. In other words the amount of water actually transferred is given by the rate of water flow multiplied by the time it is flowing at that rate. Or in other words, at a certain pressure or height:

Volume of Water = Rate Of Water Flow X Time Water Is Flowing

In exactly the same way, the amount of electrical energy used or produced by something is equal to the rate at which the energy is consumed or produced (i.e. the power) multiplied by the time that the energy is ‘flowing’. Or in other words:

Amount Of Electrical Energy = Electrical Power X Time

Which brings us to the exciting topic of Energy...

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