Application Note

3 Fluke Corporation How do you measure energy consumption?
Timing
Here’s the last complication. You
can only measure energy as the
work that your electrical system
delivers to your loads, and that
takes time. You can estimate
what energy usage will be by
observing power use for a short
period of time. Using that infor-
mation you can project longer
term energy usage with some
simple math.
Example: A 100 watt light
bulb burning for one hour con-
sumes 100 watt-hours of energy.
That same bulb would use 100 x
24 x 365 = 864,000 watt-hours,
or 864 kWh, over a year.
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Printed in U.S.A. 11/2008 3399367 A-EN-N Rev A
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Figure 4. PowerLog “Power” view. In this view of the 30-day load graph we can view the kW
and kVAR for each phase and total. From here we can identify our maximum values along with
time and duration.
Figure 3. PowerLog “Energy.” This 30-day load study graph allows us to view the kWh and
kVARh for each phase and total. An averaging period can also be selected to help match the
billing period used by the local utility (typically 15 minutes).
It gets a little more compli-
cated with motors, variable
speed motor drives, and com-
puters, but if you measure the
power usage for one hour and
then apply some assumptions to
the results, you can estimate the
energy usage for a month or a
year, provided the rate of energy
usage stays the same. The other
option is to do a 30-day load
study with a power logger. That
will get you the results shown in
Figures 3 and 4, and an absolute
understanding of your power
consumption over time.
Getting started
Ready to measure power? Using
your power quality tool, con-
nect your voltage and current
probes to your phase(s), and
start monitoring. Check your
Power (kW), Demand (kVA),
and the resulting Power Factor.
High PF is a good thing. Then
check your detractors, VARs and
Harmonics. If they are both low,
then your power supply is pretty
pure and you should be running
relatively efficiently. In terms of
energy consumption, kW and
KVA are the values to compare
over time, as you make changes
within your facility to reduce
consumption.
If you really want to save
money…
So yes—you could use your
multimeter to measure voltage
and current, make your calcula-
tions, and go from there. But the
whole point of energy reduction
is that for the first time, elec-
trical measurement accuracy
makes a monetary difference. If
your “energy” calculations are
inaccurate, because they don’t
account for power interferences
in your system, then you really
don’t know how much you’re
consuming, or what impact your
reduction efforts have. It’s worth
it to use at least an entry-level
power quality tool to get real
energy values, and to then track
those over time.