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Since I do not do routine electrical work I can not give you a good answer from your point of view. I have two regular Kill-A-Watts and one EZ.
I like the regular ones better for ease of use. Most functions are a single button press, with two requiring two presses.
The EZ has the advantage of memory retention which is useful monitoring something like a refrigerator or freeze. This you would do if you want to evaluate the average power consumption over a long period, like a day.
The devices are inexpensive, but also the quality is not good. One of my regular units is inoperative. The regulars do not have good low voltage operation. For this kind of instrument I would like to see it function down to at least 80 V. The EZ seems to work to about 50 V on voltage, do not know how well it works on power measurement on low voltage. One of the first ones I got only functioned down to 105 V, totally inadequate. This was replaced under warranty but the replacement only went to 100 V.
Do not plug a Kill-A-Watt into the output of a phase shift dimmer. Ok to plug a phase shift dimmer into the Kill-A-Watt output. To vary input voltage to the instrument use a Variac.
You should buy a 6 foot #14 appliance extension cord. Then replace the right angle plug with a good straight plug. Also make a holder to support the instrument at a convenient angle.
What can you do with this unit?
It makes a nice inexpensive digital voltmeter with 0.1 V resolution. This in combination with a small 1500 W heater can be useful for checking branch circuits. #14 copper wire has a loop resistance of about 0.5 ohms per 100 ft, and #12 is about 0.3 ohms per 100 ft. A 1500 W heater warmed up is about 10 ohms, or 12 A at 120 V.
A 20 ft #14 branch circuit will show a change of about 1.2 V for a current change of 12 A. This can be useful in troubleshooting bad circuits.
You can use the instrument to demonstrate the power consumption of a CFL compared to an incandescent, or LED for comparable light output.
You can compare different appliances, for example refrigerators.
If you can acquire one of the fraudulent power factor correction boxes being sold to residential customers as a power saver, then you should be able to prove is uselessness.
I have put good quality capacitors on the unit as a load and read 0 watts with substantial current. When testing capacitors be aware that a residual charge may remain after power off. The lower the capacitor loss the longer the charge remains.
With a Spague AB-1201 21-25 MFD 125 V capacitor the calculated reactance is about 120 ohms. At 120 V with the EZ the results were 1.19 A, 141 VA, 21 W, 0.13 PF. With the regular unit 1.25 A, 158 VA, 23 W, 0.15 PF.
With a good quality 2.25 MFD oil filled capacitor the calculated reactance is 1200 ohms. The EZ read nothing for I, W, and VA, and 1.0 for PF. The regular unit read 0.11 A, 13 VA, 0 W, 0 PF,
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