The correct way to quote electrical measurements
The Sunday Nation
Nairobi,
24 February 2013
The speed versus
velocity discussion in last week’s article elicited an interesting
question: what does the speedometer of a car display – is it speed or
velocity? When you think about it, you realize that it give the pace of
motion at every instant. Therefore, the figures displayed are
instantaneous velocities. It should actually be called a velocitimeter,
but that would be a tongue twister, wouldn’t it?
Another measurement
unit that confuses many people is the kilowatt (kW)…and its elder
brother, the megawatt (MW). When Kenya Power says that electricity
demand in the country is 1,000MW, is that per day, per month or per year
or what? And when we say generator produces 500kW, over what duration
are we talking about?
The confusion arises
because many people do not realize that power is itself a rate: it is
the amount of energy produced (or consumed) per second. Therefore, 500kW
means 500,000 units of energy per second – remember that kilo means
1,000 so 500kW is 500,000W.
Does that then mean
we should be saying 500kW per second? Of course not! Saying that is
tantamount to saying 500,000 energy units per second per second!
To fully appreciate
this, imagine a car moving at, say, 60km per hour; can you say that it
is doing 60km per hour per hour? Of course not! Unless you were talking
about its acceleration… in that case the statement would mean that,
every hour, the speed (or is it velocity?) of the car increases by
60km/h.
Thus, starting from a
stationary state, it would accelerate slowly and reach 60km/h in one
hour; 120km/h after the second hour and so on. But cars pick up speed
much faster than that – 5km/h to 10km/h per second.
In the case of power,
talking about kilowatts per second also implies that the power is
changing. Ordinarily however, are rarely concerned with the rate of
change of power. They are usually only interested in the peak values.
Therefore, the oft
quoted 1,000MW demand value is actually the peak power demand. That is,
at the time of day when the highest number of consumers are switched on.
This occurs at about 8pm every day and then it eases off to below 750MW
after midnight.
To convert from power
back to energy, we multiply the watts by time. Strictly speaking, the
time should be in seconds, but that would produce very large numbers.
Therefore, the common method is to use hours. Thus your electricity bill
shows the number of kilowatt-hours (kWh) you have consumed during the
month.
Note that these are
kilowatt-hours and NOT kilowatts PER hour. It is multiplication, not
division! Drawing a parallel with motion; when speed (or velocity) is
multiplied by time, the result is the distance covered. Thus if a car
maintained a constant 60km/h for three hours, it would traverse over a
distance of 180km (=60 x 3).
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