11546-eng-01-a-how do you measure energy consumption

8/13/2019 11546-Eng-01-A-How Do You Measure Energy Consumption

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Application Note

Core components ofenergy

If you havent measured energybefore, take a minute to under-stand how it differs from voltsand current. When we talkabout the energy supplied by

the utility, were talking abouttwo primary components: powerand demand.

Power, kW, is commonlymeasured in Watts, which indi-cate the rate at which energyis expended in one second.Watt-hours describe the totalenergy expended over othertime periods, such as a month,as recorded for ac energy useby our electrical utilities. Watt-hours measure actual work, suchas heating or cooling buildings,

moving objects or liquids, etc.Demand, kVA, measures thetotal requirement that a cus-tomer places on the utility todeliver voltage and current,without regard to the efficiencyof that delivery or whether ornot it does actual work.

Now lets start measuring. Usea regular digital multimeter withan accessory current probe tomeasure the voltage and then

How do youmeasure energy

consumption?

the current, and then multiplythe two values to get demandkiloVoltAmperes (kVA.)

This works for a simple singlephase circuit where the loadremains stable for the periodof the two measurements. Fora real-life load, we need toaccount for a few other elements.

Power factor, imaginarypower, and harmonics

Power factor. If the circuit isoperating at 100 % efficiency(which rarely happens), thendemand is also a measure ofpower. In reality, power (kW) isusually less than demand (kVA).The difference, kW/kVA, iscalled power factor (PF). Utilitiesoften collect a penalty charge

if PF falls below 0.95. Someutilities are setting the bar evenhigher. Remember: Low powerfactor is bad; high power factoris good.

The typical industrial or com-mercial facility uses three phaseenergy distribution, and thenuses that energy in a numberof waysto provide heating,operate three phase motors andmotor drives, or handle single

With energy costs high, and getting higher, many

facilities are trying to reduce their energy consumption.

Most have not previously analyzed their monthly energy

usage, traced it to operational components, or broken

out the utility fees. And until you understand how youre

using energy, its hard to make smart decisions on how

to reduce consumption.

With power, demand and a power quality tool

Using a Fluke 1735 Power Logger to log power consumption at achiller to determine equipment efficiency.


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2 Fluke Corporation How do you measure energy consumption?

Figure 1.Fluke 435 energy screen. The Energy tab allows you totrack accumulated kWh, kVAh, and kVARh.

Figure 2.Fluke 435 power screen. This is a real time view of all thecommon power calculations. The kW, kVA, kVAR, PF, dPF, along withVrms are displayed for each phase and total. The symbol to the rightof kVAR indicates whether the load inductive or capacitive.

phase loads such as computersand lighting. Three phases makeit harder to measure power orenergy usage, particularly if youplan to use improved efficiency

to reduce energy use.Imaginary power: Volt-AmpsReactive (VAR)is a strangetype of current flow that pro-duces no work, but is presenton your electrical distributionsystem. Its part of the differencebetween power and demandand thus contributes to loweringpower factor. Its usually causedby motor inductance, and isgreater when those motors arenot loaded to their full capacity.A constant speed motor driv-

ing a large air movement fanis an example where mechani-cal dampers have been used toregulate air flow, making a fanless efficient. This also reducesthe load on the drive motor andincreases imaginary power inthe electrical supply system.

Many facilities opt to changetheir motor supply from directline drive to an adjustable speedmotor drive, so that they canoptimize the motors opera-tion and speed to its load. That

optimization uses energy moreefficiently in the fan and motorand increases power factor.

Harmonic currents, reflectedback into the supply system, areproduced by the input rectifierloads of adjustable speed motordrives, computers, and similar

electronic devices. Harmonicsalso reduce power factor.

How to measure power

To measure real power, we needa meter that can simultaneouslymeasure voltage, current, andall the stuff mentioned abovethat lies in-between, over a onesecond period. A digital multim-eter cant do that. The solutionlies in a power quality tool.

Depending on the make andmodel you select, you can test

single phase, split-phase, threephase (3 wire or 4 wire) mea-surement configurations, andmeasure or record, V, A, W, VA,VAR, PF and Harmonics. Someof the recording models alsoprovide the means to recordmeasurements over time toreport the energy readings usedby the utilitykWh, kVAh andkVARh (see Figures 1 and 2).

The good news here is thatthese tools will account for allof the issues mentioned above

and accurately report energy usewhen it happens, as a functionof instantaneous voltage andcurrent measurements over time.

Setting up logging intervals.


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3 Fluke Corporation How do you measure energy consumption?

Timing

Heres the last complication. Youcan only measure energy as thework that your electrical systemdelivers to your loads, and thattakes time. You can estimatewhat energy usage will be byobserving power use for a shortperiod of time. Using that infor-mation you can project longerterm energy usage with some

simple math.Example: A 100 watt lightbulb burning for one hour con-sumes 100 watt-hours of energy.That same bulb would use 100 x24 x 365 = 864,000 watt-hours,or 864 kWh, over a year.

Fluke Corporation

PO Box 9090, Everett, WA 98206 U.S.A.Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa +31 (0) 40 2675 200 orFax +31 (0) 40 2675 222In Canada (800)-36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116Web access: http://www.fluke.com

2009Fluke Corporation.Specifications subject to change without notice.Pub_ID: 11546-eng Rev 01

Modification of this document is not permittedwithout written permission from Fluke Corporation.

Fluke. Keeping your worldup and running.

Figure 4.PowerLog Power view. In this view of the 30-day load graph we can view the kWand kVAR for each phase and total. From here we can identify our maximum values along withtime and duration.

Figure 3.PowerLog Energy. This 30-day load study graph allows us to view the kWh andkVARh 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, variablespeed motor drives, and com-puters, but if you measure thepower usage for one hour andthen apply some assumptions tothe results, you can estimate theenergy usage for a month or ayear, provided the rate of energyusage stays the same. The otheroption is to do a 30-day loadstudy with a power logger. That

will get you the results shown inFigures 3 and 4, and an absoluteunderstanding of your powerconsumption over time.

Getting started

Ready to measure power? Usingyour power quality tool, con-nect your voltage and currentprobes to your phase(s ), andstart monitoring. Check yourPower (kW), Demand (kVA),and the resulting Power Factor.High PF is a good thing. Thencheck your detractors, VARs andHarmonics. If they are both low,then your power supply is prettypure and you should be runningrelatively efficiently. In terms ofenergy consumption, kW andKVA are the values to compareover time, as you make changeswithin your facility to reduceconsumption.

If you really want to savemoney

So yesyou could use yourmultimeter to measure voltageand current, make your calcula-tions, and go from there. But thewhole point of energy reductionis that for the first time, elec-trical measurement accuracymakes a monetary difference. Ifyour energy calculations areinaccurate, because they dont

account for power interferencesin your system, then you reallydont know how much youreconsuming, or what impact yourreduction efforts have. Its worthit to use at least an entry-levelpower quality tool to get realenergy values, and to then trackthose over time.

11546-eng-01-a-how do you measure energy consumption

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