The grade school purchased a load (watts and amp) tester. It is rated for 120 V & 60Hz. It reads a 60 watt incandescent bulb at 59 watts. Close enough. It reads a 13 watt fluorescent bulb at 100 watts. I am sure this is due to the frequency increase from 60 Hz to 10KHz or more in the fluorescent bulb.

Do you know the calculations that show this result? Or can you explain why the load tester reads 100 watts for a 13 watt fluorescent bulb?

I want to be able to explain to some 8th graders why this happens and I want to say more than it is because of the increased frequency in this type of bulb.

Thanks for your help.

GH, how did you measure the compact fluorescent lamp? Did you insert the tester probes in the lamp socket? Tell us more about the tester... manufacturer, model...

It sounds like you are suggesting that the tester does not output a true watt reading, that maybe it assumes a 60 Hz frequency? Do the specifications of the meter tell you this?

I agree with hfcarson - we need to know what model you are using and how you are using it. Most "load testers" are designed to test the AC power available on the line, not the power used by a connected applicance. If what you have is actually a power meter it should be measuring the current draw on the 110 line and from that determine power consumption by the applicance. The fact that there may be an internal power supply circuit inside the device operating at a different frequency should make no difference - there is stuill 60Hz on the 110 line..

Good questions. There was virtually no identification on the unopened package I got. There were no instructions but the printing on the back of the device (a short cord with a plug on one end and a receptacle on the other with a switch (amps or watts) and a small digital read-out window). The marking on the back says 120 Volts, 60 cycles and 1850 watts. This suggests to me the item is only rated for 120 volts at 60 cycles and don't plug your huge motor into it or your space heater.

I tested many things around the house with the following readouts for compact fluorescent bulbs: 7 W reads 26W, 9W reads 39W and 23W reads 109W. The rate of change is not linear but there must be some relationship between these numbers.

There is something in these bulbs that tricks the tester into reading the wrong output. The voltage goes up to about 250v in the bulbs and the frequency goes up to 10-20kHz so what comes out of the bulb is much different from what goes in (120v and 60Hz). I tested 15-20 different items in the house and all of them read close to what would be expected so I am sure the item is working as printed on the back. There is something with the construction of these bulbs that affects the readout and (from my way of thinking) it has to be either the voltage change, frequency change, or both. I just don't know how.

At some point very high frequency will trick an instrument into thinking an open is really a short but I am not sure if this is high enough or if that is involved at all.

Any light you can shed on all this is certainly appreciated.

The name on the item is AmWatt (tm) Appliance Load Tester made by Reliance Controls. I am sure there is a picture on the web of this device.

Again, thanks for your input.

Is this it? See:

THP103 The Home Protectors | Product Details | Reliance Controls Corporation

Note that this unit is rated from 125 - 1875 watts, so when connected to an appliance drawing less than 125 watts it may give an incorrect reading. This unit can measure current as well as power draw - I'm curious what it measures for current draw with the CFL?

And again, the CFL operates at 60 Hz and 120 volts. You say that "what comes out of the bulb is much different from what goes in (120v and 60Hz)" but that's not correct. Both sides of the line powering the bulb operate at the same frequency as provided by the power utility - 60 Hz.

I agree with ebaines, loads less than 125 watts may give poor results...

... by the way, Ohm's Law is a good classroom subject. Basic physics and math that is strikingly accurate... I used to teach the fourth year electrical apprentices and Ohm's Law was a subject that we danced around the whole year!

Here is a response I got from a friend at my old place of employment. Let me know what you think.

"I suspect the watt meter is only accurate for primarily resistive loads, that is where the phase angle between the applied ac voltage and the current drawn by the device is close to zero. This is also referred to as power factor, and it could be visualized with an oscilloscope. A regular light bulb is just a resistive heater, the CFL has circuitry to create a high voltage pulse to ionize the internal gas. It is a more complex load. It might be better to use an LED bulb. The problem with low power factor loads is that they require the power delivery infrastructure to carry the additional current to deliver the power. Power companies penalize users of heavy motors unless they install power factor correction devices. "

I tested some small draw devices and they read well within what was expected. See below.
Item Tested Reading in watts
Wireless phone dock less then 1

iPad charger (stand by) 1

TV Cable box (start up) 70
TV Cable box (stand by) 5

CD player (stand by) 55

Sewing machine (stand by with light) 26

Digital clock 1

Electric tooth brush dock 2

60 W incandescent bulb 59
60 W incandescent bulb 59

Skill saw (no load) 1050

Here is another interesting bit of information. I took the stated wattage (7,9,23) and divided it into the reading produced by my instrument with the following results:

Stated Reading S/R
23 109 4.74
9 39 4.33
7 26 3.71

These are too close not to have the same thing happening each time I take a reading.