Why am I getting strange results when using a power meter to measure CFLs?

Question

Over the years we have replaced just about every bulb in the house with a CFL. But here is the problem. Our electric bill is going up and not down. When I moved here 3 years ago we spent about $140-160. As of this summer we topped $210. Electric rates have raised of course but that doesn't explain why our physical electric usage is has gone from 800 to 1100+ kwh

I purchased a Reliance Controls THP103 AmWatt Generator Appliance Wattage/Amps Load Tester to see if it was something that was busted like the compressor on the fridge that always makes noise. Sears says no and my meter suggests no.

After testing ever in the house I decided to test the CFL's. Different brands, some generic some branded. Meter said that a incandescent bulb of 40w was using 39 and a 60w was using 59. But it also said that an 11w CFL was using 45w and all of the other CFL's had similar readings.

Is this normal?

Answer 1

Why your various meters don't do a good job measuring power being used by various devices is due to an AC Circuit phenomenon known as Power Factor.

Power Factor The ratio of real power to apparent power.

Real power is the work done, apparent power is the product of the current and voltage in the AC circuit.

Capacitive and inductive loads cause a phase change between the current and voltage.

Inductive loads cause current to lag voltage and capacitive loads cause current to lead voltage. Think of two sine waves, one representing current and the other representing voltage that get offset by up to 90 degrees depending on the type of load. This phase shift messes up the reading of the real power consumed by the circuit. And if the power meter does not measure power factor and correct for it, you basically have a good door stop.

If you have a lot of motors in a plant, you often have to add capacitors on the power line coming into the building so you don't mess the power grid up. It is done to reduce the phase difference between current and voltage.

So, if you're going to measure AC Power, the device must measure True Power (Watts), Apparent Power (VA), Power Factor (%PF) in addition to Voltage (RMS), Current (Amps) and Frequency (Hz). Once you have the True Power and Time, you can read kWH consumed.

Answer 2

Seems the issue is with Reliance testers:

ElectricianTalk.com has this to say

A while back I [posted] that I had bought a lot of cfl lamps. My electric bill did not seem to change all that much so I decided to test some of them. Here are some of the results.

15 watt cfl P=62watt
15 watt spot P=73 watts
13 watt mini spiral P=51 watts

I am using a Reliance Controls load tester.

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On Amazon there is this review

Does NOT work with compact fluorescent lights!, May 28, 2011

This review is from: Reliance Controls THP103

Anything that uses an electronic ballast (light the compact fluorescent lights - CFLs) will not work with this device. That's because the device looks at the peak current to figure out the watts. For example, a CFL that actually uses 13 or 14 watts will erroneously show up as using 65 watts.

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On HomeDepot.com, another review

Highly inaccurate,. When ever I would read CFL bulbs the amp and watt reading was unsually high. I first I thought I had bad CFL's. My incandescent bulbs and a heater all read correctly.

I then spoke to Reliance Controls and they confirmed that due to the fact it not a true RMS meter it can not accurately calculate some loads, one being CFL's.

Answer 3

Directly from Reliance Controls

CFL and LED lighting have irregular current draws and in turn, irregular (non-linear) waveforms. These loads cannot be measured by this device accurately, sorry. Primarily the THP103 is intended for measuring medium-large sized loads to help facilitate load management in a backup generator/transfer switch application, the accuracy range is about 125 watts - 1875 watts (1 amp - 15 amps). Although it can measure loads under 1 amp quite efficiently, accuracy will be lost the smaller the load is and closer to zero. But in this case, only a meter with True RMS capability will be able to accurately read CFL and LED lighting. The applied voltage changes thousands of times per second in an LED or CFL, therefore a meter with a powerful computer processer is required to sample the waveform tens of thousands of times per second to create a sufficient sample. The THP103 is an RMS meter and as most loads in an average home are resistive with linear waveforms, a meter like this can be used for almost all appliances. We chose not to create a True RMS meter due to the costs associated with the high speed computer processing, the device would probably cost above 100$ and this price level wasn't in line with the intention of the THP103. Additionally, when this was first designed around 10 years ago the use of CFL and especially LED lighting was extremely minimal.

We have performed some testing on CFL's at our high current lab with expensive test equipment, and they test pretty accurately in relation to their advertised ratings, it should be safe to go by the ratings without having to verify that with any current measuring device.