I got a P3 Kill A Watt as Christmas present. Besides being able to measure voltage, current and kilo-watt-hours, it can measure power factor as well.

As we know, the power factor of a purely resistive AC circuit is 1.0. Typical household electronics contain inductive or capacitive elements and thus have a non-zero phase angle between the current and voltage. This non-zero phase angle results in a power factor less than 1.0.

To improve power transmission efficiency, we would like to have the collective loads to appear as close to purely resistive as possible. If the power factor of the loads is significantly less than 1.0 then significant amount of energy is wasted during power transmission due to power line resistance.

So as a curiosity, I decided to measure the power factors of a few compact fluorescent lamps. Since one of the primary reasons people chose to use CFL’s is to save electricity, I had expected that the power factor of these CFL’s to be relatively high. But to my surprise, typical CFL’s found common in Home Depot and Walmart have rather low power efficiencies. For the CFLs I measured (ranging from 10W to 40W) they all have a power factor of just above 60%.

Clearly, more energy could be saved if the power factor of these CFLs are higher. But manufactures see no need of adding cost to their final products by adding power factor correction circuitry. After all, typical consumers only care about how much electricity they can save. Whether the power factor is 0.6 or 0.8 it doesn’t matter as the savings are pretty much the same regardless of the power factor. As for the environment it is a totally different story: the higher the power factor, the less energy it is wasted during power transmission and thus the better it is for the environment.

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3 Thoughts on “Power Factor of a Compact Fluorescent Lamp”

  • I don’t think one can divide a device’s delivered power by apparent power, nor use its PF, to directly get its efficiency. True, CFLs have low PFs but I’d guess the power company, aside from attempts at correction on their end, won’t do much about it in homes because 1. the cost in changing the installed base of residential energy meters that can’t measure PF, 2. pure reactive power, aside from hysteresis loss, eddy current loss, (and skin effect if there’s enough powerline distortion) is energy stored and returned — I think what’s more significant, as you stated, is the I-squared losses in the transmission to relatively loss-free reactive components through resistive ones. Here, too, the power company must provide adequately sized wire and delivery components to transmit the higher current resulting from low PF. 3. since house wiring is not that long and there aren’t usually big motors or transformers, much higher gains are attained by focusing power quality and PF stringency on big companies, which do pay for lower PFs. For safety, load calculations and your wiring should use the current in the VA rating.

    I think companies lobbied the regulatory bodies and were able to exempt some low volume products from expensive PF correction. The greater concern is for problems from distortion.

    Out of the realm of CFLs, PF correction introduces a different set of quality and safety problems. There’s increased distortion which, in 3 phase systems, can cause dangerously excessive currents in the neutral. PF-correction capacitors can cause this, by their resonance with the line, as well as large, undesired voltage fluctuations, in switching environments. These also cause transients, arcing, larger upstream currents, and RFI/EMI, and may trip GFIs. Another personnel safety issue may be raised with the placement of the capacitors. Got most the above from one of Ralph Morrison’s great books on grounding. Also have a book by Ray Mullin which does a suboptimal job in its PF-efficiency examples.

    I think that past energy company debacles have shown that they may be more interested in the monetary leverage in selling services and financial instruments like futures and derivatives based on energy than selling actual energy; and than in the environment. Your point is well-taken: environmentally, in the aggregate, this is a poor model and exhibits poor attitude on all sides, and customers won’t care if they aren’t charge extra for low PFs.

    Thanks for your the post. I too recently got a Killawatt & have had fun with it. It was well worth it.

    • To clarify (or maybe to cloud the issue) on my post last night:

      When mentioning “distortion,” I meant total harmonic distortion on the powerline.

      And “pure” reactive power would not include iron core, hysteresis, or eddy current losses. What I shd have
      said was, aside from real losses resulting from reactive components, there can also be resistive loss in the
      “delivery” of the current to reactances. Electrical distribution is designed with Apparent Power in mind. This is
      the clear road to equipment and personnel safety.

      Next, from device specs I’ve seen online and in stores, although there’s data (for example, a PGE Energy Center
      paper talks about Ballast Factor, Lamp efficacy, Lamp/ballast system efficacy, and Ballast Factor), these are not
      a totally clear representation of classic electric efficiency parameters in the calculation of Pout/Pin.

      It seems that a known PF, combined with load-side circuit parameters like impedance, resistance, reactances,
      true power, or reactive “power,” can render much data, but unless one has true power and not volt-amps entering the
      system, electrical efficiency can’t be derived. And since folks are interested in equivalent luminosity and if they can
      lower energy consumption for a given light output, this mathematical equation won’t matter.

      As to the salient points you made, they are quite valid.

      PF can be “transparent” to customers even if they save energy and they care about the earth.

      And if CFL systems and their delivery systems end up using less total energy, for the same effective luminosities in
      homes, then I feel THAT is better for the environment — a higher PF alone will not accomplish that unless resistive
      current is reduced. To improve efficiency AND to save electricity we would like unity PF but we’d also simultaneously
      like less total current.

      Lastly I’d add that electric code limits residential lighting circuits to 80% of rated value,
      e.g., if there are lights on a 20-A breaker and wiring, then circuit current should be limited to 16A.

      BTW Leonard-Energy has a poster stating he successfully implemented PFC in his magnetic ballasts, washing machines,
      and fridge, using one capacitor.


  • I’ve looked at this again and I may be wrong — power factor in sinewave loads may equate to efficiency. But I think if this is so, then that would be very misleading w. respect to actual energy wasted and effect on the environment. I made some measurements with the Kilo-watt device: its measurements of current, volt-amps, and power factor, all conform. But I still think that this current, while measureable on newer metering meters, is not totally “wasted,” though the utility needs to size its transformers and wires to deliver this instantaneous current. When harmonics are thrown in the mix it gets a bit complex.

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