CHAPTER 1: THREE-PHASE POWER MEASUREMENT POWER, ENERGY AND DEMAND
EPM 2200 POWER METER – INSTRUCTION MANUAL 1–7
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 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.
The data from Figure 1.7 is reproduced in Table 1.2 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).
Figure 1-7: Power Use over Time
Table 1.2: Power and Energy Relationship over Time
Time Interval
(minute)
Power (kW) Energy (kWh) Accumulated Energy
(kWh)
1300.500.50
2500.831.33
3400.672.00
4550.922.92
5601.003.92
6601.004.92
0
10
20
30
40
50
60
70
80
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Time (minutes)
sttawolik
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