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Figure 1.8 shows another example of energy and demand. In this case, each bar represents the energy consumed in a
15-minute interval. The energy use in each interval typically falls between 50 and 70 kWh. However, during two in-
tervals the energy rises sharply and peaks at 100 kWh in interval number 7. This peak of usage will result in setting
a high demand reading. For each interval shown the demand value would be four times the indicated energy reading.
So interval 1 would have an associated demand of 240 kWh/hr. Interval 7 will have a demand value of 400 kWh/hr.
In the data shown, this is the peak demand value and would be the number that would set the demand charge on the
utility bill.
As can be seen from this example, it is important to recognize the relationships between power, energy and demand
in order to control loads effectively or to monitor use correctly.
1.3: Reactive Energy and Power Factor
The real power and energy measurements discussed in the previous section relate to the quantities that are most used
in electrical systems. But it is often not sufficient to only measure real power and energy. Reactive power is a criti-
cal component of the total power picture because almost all real-life applications have an impact on reactive power.
Reactive power and power factor concepts relate to both load and generation applications. However, this discussion
will be limited to analysis of reactive power and power factor as they relate to loads. To simplify the discussion,
generation will not be considered.
Real power (and energy) is the component of power that is the combination of the voltage and the value of cor-
responding current that is directly in phase with the voltage. However, in actual practice the total current is almost
never in phase with the voltage. Since the current is not in phase with the voltage, it is necessary to consider both the
inphase component and the component that is at quadrature (angularly rotated 90 degrees or perpendicular) to the
voltage. Figure 1.9 shows a single-phase voltage and current and breaks the current into its in-phase and quadrature
components.
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Electro Industries/Gauge Tech
Kilowatt-hours
100
80
60
40
20
Figure 1.8: Energy use and demand
Figure 1.8: Energy Use and Demand
Doc# E149701
Intervals
1-8
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