The layman often uses the words energy and power interchangeably. However, in science, these two words have very distinct meanings. To understand what is meant by the terms energy and power, an analogy with the flow of water may be used.
Consider a tap from which there is a flow of water of 5 liters per minute. The volume of water delivered in a particular amount of time is equal to the flow multiplied by time. This relationship is given in equation form below. Note that we can also say that the flow is the rate at which volume is delivered.


Thus, if the tap is left open for 2 minutes, the total volume of water which will be delivered is 5 * 2 = 10 litres. The connection between energy/power and volume/flow is simple: we can think of energy to be like volume and power to be like flow.
With this analogy in mind, we get the following two equations.

Therefore, just as flow is the rate at which volume is delivered; power is the rate at which we use energy.

Watt, kilowatt and kilowatt-hour

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In the preceding discussion, we measured the volume of water in terms of liters. Energy too, is measured in terms of “something” and this something is called the joule (symbol J). You may already have come across the joule as it found on certain food packages where it gives the energy content of the food product. For example, the label on a box of cheese informs us that this product contains about three million joules of energy.

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WATT KILOWATT
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We next turn our attention to the units we use to measure power. Remember that we said that power is the rate at which we use (or produce) energy. Therefore, power is measured in joules per second (compare with the litres per second for flow). However, power is such a useful and important quantity that it is given a special unit of its own- the famous Watt (symbol W). A power of 1 W is thus the same thing as a power of 1 J/s (joule per second).
A power of 1 W is quite small and it often quickly becomes cumbersome to work with. Therefore, for most applications, we do not use the watt but the kilowatt (symbol kW). 1 kW is equal to 1000 W just as 1 kilometre is equal to 1000 metres. Note carefully that the “k” in kW is a small letter. It is incorrect to write KW. All too often we find this mistake in our newspapers. In the same vein, another unit of power worth mentioning is the megawatt (symbol MW). The public often comes across the megawatt in the media as it used to describe the capacity of power stations. We emphasize here the capital letter “M” in MW. If someone were to write mW, he would infact be referring to the milliwatt which is equal to one thousandth of a watt (0.001 W). Thus, statements which describe power stations capacities in mW are clearly wrong.

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The last item to be covered is the kilowatt-hour which has the symbol, kWh. 1 kWh of electrical energy is the amount of energy an apparatus rated at 1 kW consumes in 1 hour. More generally though, the amount of energy an apparatus consumes is given by:
Amount of energy in kWh = Rating in kW of apparatus Number of hours in use
For example, if a 1.5 kW oven is operated for 2 hours, the amount of energy in kWh that it consumes is given by 1.5 2 = 3 kWh. As a further example, what is the number of kWh an 800 W kettle consumes if it is on for 15 minutes? Since we have been asked the amount of energy in kWh, we first convert the 800 W into kW and the 15 minutes in hours as required in the above equation. Thus, 800 W = 0.8 kW and 15 minutes = ¼ hour. The amount of energy in kWh is then equal to 0.8 ¼= 0.2 kWh.
Finally, it should be noted that the kilowatt-hour is in fact the “unit” (commonly known as “unité”) which we find in our CEB bill. Thus, a consumption of 140 units actually refers to 140 kWh of electrical energy.

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KILOWATT HOUR