Staggered Start and Demand Savings
Richard R. Vaillencourt, PE
Canterbury Energy Engineering, LLC
Abstract
When a motor is started, it can draw from 300% to 600% of its nameplate current for a
few (1 – 3) seconds. This inrush current has been identified as a cause of high peak
electrical demand charges. However, because of the way that the peak demand is
measured by electric utility meters, the inrush current has an insignificant impact on the
measured peak electrical demand for a facility. Therefore, taking pains to stagger the
start of electrical motors simply to minimize the combined impact of inrush current on
demand charges is a myth. Timing the start of individual loads to avoid coincident longterm startup peaks is the answer to peak demand control. This paper will explain
electrical peak measurement and how to really avoid unnecessary peak demand increases.
Introduction
Peak electrical demand is measured by utility companies and billed separately to
accommodate the capital investments required to ensure that all customers will receive all
the electricity that they want at any time. The two most common measurement periods
are fifteen and thirty minutes. The highest electrical rate of consumption (demand) for a
fifteen minute period is charged for the entire month (sometimes for eleven months if
there is a ratchet charge). It is economically important to keep this value as low as
possible.
The first rule is to avoid the simultaneous operation of equipment whenever possible.
Energy engineers in the past have recommended waiting several minutes between starting
large motors so that the 300% inrush current from each motor does not add to each other
to result in an unnecessarily high peak demand. While it is better for the electrical
distribution system to avoid cumulative inrush currents, the affect of inrush current on the
measurement of the peak demand is negligible because of the way the peak demand is
measured.
Discussion
Peak Demand
It is often stated that the peak demand is the highest fifteen-minute demand for the
month. The correct statement is that it is the highest average fifteen-minute demand for
the month. The demand is measured by accumulating kWh (energy consumption) over a
fifteen-minute period and then dividing by the number of hours (0.25 hours) to obtain the
average demand. For example: 100 kWh accumulated in a fifteen minute period:
100 kWh
 400 kW
0.25 hours
Staggered Start and Demand Savings
So the real question is how much does inrush current affect the accumulated kWh?
Assuming the following motor parameters (related directly in kW to keep things simple):

Motor nameplate load: 50 kW

Motor inrush:
o Extra kW:

Inrush duration:
300%; 150 kW
100 kW
3 seconds
There appears to be a significant demand increase during motor startup. An extra 100
kW is drawn from the system whenever this motor starts. But it only lasts for three
seconds! So the additional kWh is extremely small.

3 seconds = 0.00083 hours

100 kW X 0.00083 hours = 0.083 kWh
Therefore, the inrush current will increase the accumulated 15-minute kWh from 100 to
100.083 kWh. Therefore the affect on the peak demand is:
100.083 kWh
 400.33 kW
0.25 hours
The increase due to the inrush current is less than 0.1% (0.083%).
Staggered Start
So when does staggered starting equipment make sense? If you have multiple pieces of
equipment that will operate at full load during a lengthy startup period and then “coast”
back to a lower operating load. For instance: multistage air conditioning units.
Suppose that you had three, two-stage AC units with 25-kW/stage. The total unit load is
50 kW. During morning cool-down, the AC unit will operate at full load (50-kW) until
the space is satisfied. Once the space is at occupied temperatures, the second stage will
shut off.
Therefore, starting one unit at a time, then waiting for it to drop back to single-stage
operation will reduce the peak demand caused by starting these units.
Startup without staggering
#1
#2
1st 15-min
50
50
2nd 15-min
25
25
3rd 15-min
25
25
4th 15-min
25
25
#3
50
25
25
25
Total
150
75
75
75
Startup with staggering
#1
1st 15-min
50
2nd 15-min
25
3rd 15-min
25
4th 15-min
25
#3
0
0
50
25
Total
50
75
100
75
Canterbury Energy Engineering, LLC
#2
0
50
25
25
2
Staggered Start and Demand Savings
The peak demand is still higher than the “run” demand for all three units, but there will
be a savings of 50-kW by not running all three in “start-up” load at the same time.
Another minor consideration is that you will have to start the first and second units before
they are needed (if they are all needed to be cooling by the same time in the morning) so
that the third unit will be able to come on when it is needed. In this case, the first unit
will need to start 30-minutes earlier and the second unit 15-minutes earlier. Consider
whether an extra 30 and 15 minutes run time every day will cost more than the potential
demand savings.
Canterbury Energy Engineering, LLC
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