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1.2: Power, Energy and Demand
It is quite common to exchange power, energy and demand without differentiating between the three. Because this
practice can lead to confusion, the differences between these three measurements will be discussed.
Power is an instantaneous reading. The power reading provided by a meter is the present flow of watts. Power is
measured immediately just like current. In many digital meters, the power value is actually measured and calculated
over a one second interval because it takes some amount of time to calculate the RMS values of voltage and current.
But this time interval is kept small to preserve the instantaneous nature of power.
Energy is always based on some time increment; it is the integration of power over a defined time increment. Energy
is an important value because almost all electric bills are based, in part, on the amount of energy used.
Typically, electrical energy is measured in units of kilowatt-hours (kWh). A kilowatt-hour represents a constant load
of one thousand watts (one kilowatt) for one hour. Stated another way, if the power delivered (instantaneous watts)
is measured as 1,000 watts and the load was served for a one hour time interval then the load would have absorbed
one kilowatt-hour of energy. A different load may have a constant power requirement of 4,000 watts. If the load were
served for one hour it would absorb four kWh. If the load were served for 15 minutes it would absorb ¼ of that total
or one kWh.
Figure 1.7 shows a graph of power and the resulting energy that would be transmitted as a result of the illustrated
power values. For this illustration, it is assumed that the power level is held constant for each minute when a mea-
surement is taken. Each bar in the graph will represent the power load for the one-minute increment of time. In real
life the power value moves almost constantly.
The data from Figure 1.7 is reproduced in Table 2 to illustrate the calculation of energy. Since the time increment of
the measurement is one minute and since we specified that the load is constant over that minute, we can convert the
power reading to an equivalent consumed energy reading by multiplying the power reading times 1/60 (converting
the time base from minutes to hours).
e
Electro Industries/Gauge Tech
Kilowatts
100
80
60
40
20
Figure 1.7: Power use over time
Doc# E149701
Time (minutes)
1-6
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