Kill-A-Watt Watt calculator is confusing me, doesn't add up - Page 1
Forum section: electronics / beginners

author: TT_Vert
author_rank: Regular Contributor
timestamp: June 06, 2022, 01:46:24 am
content: I have a POE switch plugged in w/ the kill a watt monitoring the consumption. It shows 200mA at 120V but the Wattage shows 4W. This should show ~24W no? I'm sure I'm missing something here.Dave

author: xrunner
author_rank: Super Contributor
timestamp: June 06, 2022, 01:59:38 am
content: Quote from: TT_Vert on June 06, 2022, 01:46:24 amI have two of them but I can't remember checking on such a low current. I'll try something tomorrow and report back unless someone else answers your question.

author: IanB
author_rank: Super Contributor
timestamp: June 06, 2022, 02:32:53 am
content: Quote from: TT_Vert on June 06, 2022, 01:46:24 amWhat about the power factor? If the power factor is not 1.0 then power is not equal to volts times amps.

author: james_s
author_rank: Super Contributor
timestamp: June 06, 2022, 03:22:25 am
content: Quote from: IanB on June 06, 2022, 02:32:53 amBut with a power factor less than unity the wattage will always show higher than it is, in this case he's seeing lower.I wonder if this is a real Kill A Watt or one of the knockoffs?

author: IanB
author_rank: Super Contributor
timestamp: June 06, 2022, 04:17:21 am
content: Quote from: james_s on June 06, 2022, 03:22:25 amI think it will show lower. Kill-A-Watt devices show true power in watts. (They show the VA and the power factor separately.) If the power factor is less than unity then the reactive current will cause the current reading to increase. In this case it seems to be showing a true power of 4 W, but a total current of 200 mA. So everything seems consistent with a poor power factor.This is easy to check, if the OP would kindly indicate all the readings displayed: Volts, Amps, Frequency, Watts, VA, Power Factor.

author: bdunham7
author_rank: Super Contributor
timestamp: June 06, 2022, 04:24:47 am
content: Is this the P3 KillAWatt? If you have 120VAC 60Hz 4W 200mA, when you press some more buttons you should see ~24 VA, ~PF 0.16 and 60HZ. Your apparent power is the voltage times the current, the real power is the sum of a bunch of instantaneous calculations and PF (power factor) is real divided by apparent power.

author: james_s
author_rank: Super Contributor
timestamp: June 06, 2022, 05:07:05 am
content: Quote from: IanB on June 06, 2022, 04:17:21 amThat makes sense if he's comparing against the wattage he expected by calculating VA, I had assumed the wattage was listed on the device, you're probably right.

author: Zero999
author_rank: Super Contributor
timestamp: June 06, 2022, 11:05:04 am
content: Quote from: TT_Vert on June 06, 2022, 01:46:24 amThe Kill-A-Watt measures power, which is 4W and you've calculated the apparent power i.e. VA which is 24V.On AC, it's quite common for a load to draw more current, than expected for its power consumption. For example a capacitor connected across the mains it will dissipate no power, because the current leads the voltage by 90º, so when the current is at the peak, the voltage across it is zero and power = v*i. The capacitor charges and discharges, causing power to be transferred between the capacitor and back to the mains again. The net power dissipation is ideally zero, yet in reality it's a tiny amount due to the resistance in the wiring.In a resistor, the power is always dissipates power.Here's a simulation of a resistor and capacitor connected to the mains. The sine on the power dissipation in the capacitor goes positive and negative, showing power is being transferred to and fro between the capacitor and the mains. With the resistor, the sign is always positive.

author: xrunner
author_rank: Super Contributor
timestamp: June 06, 2022, 11:17:34 am
content: Just hooked up my Kill-a-watt to a cheap DC power supply and a 100 ohm load. I set the DC power supply output voltage to draw 200 mA (AC) as indicated on the Kill-a-watt. here's the Kill-a-watt data:124.6 VAC0.2 A13.5 W25.2 VA0.53 PFSo you can see per the explanations what is going on with watts and power factor.

author: xrunner
author_rank: Super Contributor
timestamp: June 06, 2022, 12:39:21 pm
content: Quote from: TT_Vert on June 06, 2022, 01:46:24 amHere I simply lashed up an AC voltage from a Variac to a 100 ohm resistor. I turned up the output voltage so that the Kill-a-Watt read 0.1 A.The Kill-a-Watt gives the following:124.9 VAC0.1 A12.6 W12.8 VA0.99 PFNow you see the Watts and VA reading are nearly identical, because the power factor is 0.99. Voltage and current are in-phase so the watts reading is what you will calculate from Volts * Amps.

author: TT_Vert
author_rank: Regular Contributor
timestamp: June 07, 2022, 03:43:17 am
content: Sorry guys, ill get to these replies tomorrow. I am going through a DC to AC inverter (kill-a-watt plugged into AC out of the inverter) if that matters. Seems one I put a higher load on it it got closer to what I had expected to see.

author: Zero999
author_rank: Super Contributor
timestamp: June 07, 2022, 08:09:44 am
content: Quote from: TT_Vert on June 07, 2022, 03:43:17 amThese sort of power meters tend to be less accurate with lighter loads.What sort of inverter is it? Does it have a pure sine wave output, or a modified sine wave? The power meter will be designed for use with a pure sine wave. I expect it should work with a modified sine wave, but it might be less accurate.

author: TT_Vert
author_rank: Regular Contributor
timestamp: June 16, 2022, 12:44:25 am
content: Quote from: Zero999 on June 07, 2022, 08:09:44 amIt's currently in the shed but it's an OLD Tripp lite. It just says "sinewave"This is itQuote from: bdunham7 on June 06, 2022, 04:24:47 amYes, actually it is.Dave

author: james_s
author_rank: Super Contributor
timestamp: June 16, 2022, 05:11:09 pm
content: That inverter is modified sine wave for sure. Some of those reviews say they've used it since 2005, compact affordable pure sine inverters are a relatively recent development.

author: Zero999
author_rank: Super Contributor
timestamp: June 17, 2022, 07:14:52 am
content: Repeat the test with it powered off the mains and compare the results.