Demand - GE EPM6000 Instruction Manual

POWER, ENERGY AND DEMANDCHAPTER 1: THREE-PHASE POWER MEASUREMENT
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 intervals 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.
1.2.1

Demand

Demand is also a time-based value. The demand is the average rate of energy use over
time. The actual label for demand is kilowatt-hours/hour but this is normally reduced to
kilowatts. This makes it easy to confuse demand with power. But demand is not an
instantaneous value. To calculate demand it is necessary to accumulate the energy
readings (as illustrated in the Power Use Over Time figure above) and adjust the energy
reading to an hourly value that constitutes the demand.
In the example, the accumulated energy is 14.92 kWh. But this measurement was made
over a 15-minute interval. To convert the reading to a demand value, it must be
normalized to a 60-minute interval. If the pattern were repeated for an additional three 15-
minute intervals the total energy would be four times the measured value or 59.68 kWh.
The same process is applied to calculate the 15-minute demand value. The demand value
associated with the example load is 59.68 kWh/hour or 59.68 kWd. Note that the peak
instantaneous value of power is 80 kW, significantly more than the demand value.
The following figure illustrates 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 intervals the energy
rises sharply and peaks at 100 kWh in interval #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.
1–10 EPM6000 MULTI-FUNCTION POWER METERING SYSTEM – USER GUIDE