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1.2
Power, Energy and Demand
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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 1 kWh.
The following figure 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 measurement 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.
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CHAPTER 1: THREE-PHASE POWER MEASUREMENT
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Time (minutes)
FIGURE 1–7: Power Use Over Time
EPM 2200 MULTI-FUNCTION POWER METERING SYSTEM – USER GUIDE
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