CHAPTER 1: THREE-PHASE POWER MEASUREMENT
EPM 6010 MULTI-FUNCTION POWER METERING SYSTEM – USER GUIDE
1–9
The data from Figure 1-7 is reproduced in the table below 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).
As in Table 1-2, the accumulated energy for the power load profile of 1-7 is 14.92 kWh.
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 Figure 1-7 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/hr or 59.68 kWd. Note that the peak
instantaneous value of power is 80 kW, significantly more than the demand value.
Table 1–2: Power and Energy Relationship Over Time
Time Interval
(Minutes)
Power
(kW)
Energy
(kWh)
Accumulated
Energy (kWh)
1
30
0.50
0.50
2
50
0.83
1.33
3
40
0.67
2.00
4
55
0.92
2.92
5
60
1.00
3.92
6
60
1.00
4.92
7
70
1.17
6.09
8
70
1.17
7.26
9
60
1.00
8.26
10
70
1.17
9.43
11
80
1.33
10.76
12
50
0.83
12.42
13
50
0.83
12.42
14
70
1.17
13.59
15
80
1.33
14.92
