How electricity is measured and sold
The Sunday Nation
Nairobi,
08 May 2011
Joseph Mwangi wants to get a better understanding of how electricity consumption is measured. He writes, “I would like to understand what constitutes a unit, how is it measured and how the resulting tariffs are calculated. For example when one says that an engine produces 30kwh, what does this mean? Is it that the engine produces 30kw each hour or each second? How many Kw make up 1MW and how many Kwh make up 1MWh?”
Let’s start with the easier part. In the International System (SI) of units, quantities are grouped in multiples of ten – in most applications, multiples of 1,000. Thus 1,000 units are called a “kilo” (k); 1,000,000 units make a “Mega” (M); 1,000,000,000 are a “Giga” (G) and so on.
Fractions of units are also grouped in a similar way, for example: 0.001 [thousandths] is a “milli” (m); 0.000001 [millionth] is a “micro” (Greek letter mu) and so on.
Therefore, if you look carefully, you will see that there are 1,000 kilo-anything in one mega-anything; and 1,000,000 kilos in one giga; and so on. This rule applies to all measurements except computer memory where the base multiple is 1024 – the reason for that is a story for another day.
Lastly, the SI symbols are case-sensitive; for example, M means mega (1,000,000) while m stands for milli (0.001). For that reason, allow me to correct Joseph as follows: kilo-watt is kW (not kw or Kw) and kilo-watt-hour is written as kWh.
With the symbolism out of the way, what then, is a kilowatt? Answer: it is 1,000 watts. And what is a watt? It is a measure of the rate of consumption or production of energy; that is the number of energy units (joules) consumed or produced every second.
It follows then, that if you multiply the number of watts by the number of seconds you should get the amount of energy (number of joules) consumed or produced. However, electricity suppliers the world over (KPLC and the like) do things slightly differently: instead of seconds, they use hours.
The end result is the same: whether you use seconds or hours the result is a measure of the energy consumed or produced. So, “when one says that an engine produces 30kWh” it means that this is the amount of energy available in the machine. This is determined by, perhaps, the size of the fuel tank.
But as Joseph asks, “Is it that the engine produces 30kW each hour or each second?” Now this question confuses two issues: the total quantity of energy available, and the rate of production. 30kWh is the total energy available in the machine.
If this particular machine is operated at the rate (and I emphasise, “rate”) of 30kW, then it will run for one hour and then run dry (of energy). However, if the rate is 15kW, then it will go for two hours before stopping… a rate of 1kW will correspond to 30h running time and so on.
Finally, a word of caution: 30kW is the energy produced per second; therefore, it is wrong to say 30kW per second!
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