Kill a watt, how accurate?

JiminDenver wrote:
Wanting to know just how much power the window shaker uses, I bought a Kill a watt. It's been to cold to test the AC but most things I have tested are under the rated watts.

The 1875w hair dryer was only pulling 1660w. The oil filled heater pulled 590w on the low 600w setting but only 1380w on the 1500w high setting. Is this normal?

having worked as a designer of test equipment and UL etc. what I can say is that when it comes to listed power ratings, UL is very picky. do NOT exceed the listed power under any conditions. Therefore, our listed power was always more than what the typical draw was. As an example the hair dryer watts probably takes into account max line voltage of near 130V, and min resistance of the coil of wire. Same with the heater.

What I've found is that the killawat is accurate enough for my needs, and probably is pretty accurately measuring the actual power draw of the devices in question under your test conditions.

Well that's good to know and I am really hoping the window shaker uses less than its rated 446w. That will explain how the solar could run it and not drain the battery.

I have 2 of the P4400s and have found both to be surprisingly accurate after cross-checking them with a few other measuring devices including a revenue meter. Many light bulbs and heaters don't come very close to their advertised wattage, plus you have to know what voltage they're rated at to even know what to expect. As an example, while the standard rating voltage is 115, almost all hair dryers are rated at 125 volts. Because of this "nobody" will ever get the rated performance, but the higher wattage looks good in the advertising. The only places I've found the P400 lacking are the frequency and power factor readings with "modified sine wave" power. Even with MSW, the other functions give what looks to be good true RMS readings.

Sounds about normal.

Dakota98 wrote:
Which model # did you purchase ?

I got the P4400 from P3 international.

Jim,
I have one of the cheaper KAW's and although I haven't calibrated it against the Fluke clamp meter, I'd say it was fairly accurate, especially for a purely resistive load. At sometime, I'll put it head-to-head with the Fluke and see if there is a difference.

No, but purchasing a 150 watt light bulb and having the meter read 150 watts +- 2% is normal. Just make sure your equations are utilizing OEM voltage versus amperage versus Kw rendered values. A pure resistance test will also reveal the meter's VA versus wattage agreement or disagreement.

Which model # did you purchase ?

So after more testing, I found that a GE 100w bulb pulls 100w constantly. Oddly I first tried a off brand 100w bulb from Mexico and it started at 96w only to drop to 83w as it heated up. I'll have to see if the light output is affected with the light meter on my phone.

A 60w equivalent Phillips LED pulled a half a watt higher than it was rated for, so there isn't a constant under measurement by the Kill a watt.