Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Kill-A-Watt meter alters runtime on UPS?

  1. Jan 20, 2013 #1
    I’m hoping someone can help me.

    I have several UPSs. Among them is a cheap Newpoint UPS. It’s something like 275W/550VA. I think the battery is dead or near-dead, was recently experimenting with it to confirm, and found some very interesting behavior.

    I have a 120V lamp with a LED bulb in it. It takes about 8W, 15VA.

    if I fully charge this UPS, hook the lamp up, turn it on, and yank the plug, the light remains on for about 3 minutes.

    here’s the kicker – if I fully charge the UPS, hook the lamp up with a “Kill-A-Watt” power meter between lamp and UPS, then pull the plug, it remains on for 24 minutes. there is no discernible difference in brightness.

    the meter itself is simply measuring usage, yes? surely it doesn’t increase runtime 7x.

    yet, this behavior is reproducible – at least with this lamp and UPS.

    further, with a 60W incandescent, it goes out immediately in both configurations. this confirms my suspicion of a bad battery, and explains why it quits in under a minute on my Verizon FiOS ONT, but doesn't add any data to the runtime conundrum.

    interestingly enough, after it fails with 60W, I can turn it back on and run the LED without plugging it back into the wall. That 60W load is too much.

    other fun fact: when the UPS is plugged IN, the meter (between UPS and lamp) shows 8W, 14-16VA usage. when the UPS is unplugged, it shows 0W, 0 VA, yet the light is on.

    can anyone explain this? I'm a biological scientist type with a working understanding of this stuff, and I know I need to replace this battery (already ordered), but what is going on with the meter?
  2. jcsd
  3. Jan 21, 2013 #2


    User Avatar
    Science Advisor

    I would guess that the Kill-A-Watt meter introduces enough of an in-series load (especially at such low power) that the battery lasts that much longer. It may also be that the reduced current draw doesn't tax the battery as much, allowing the battery to discharge deeper. In any case, lowered draw also means lower power consumption in the UPS itself.

    You may not be able to perceive much difference, but our perception of brightness is logarithmic: doubling the power of a light source does not lead to a doubling of perceived brightness (and consequently, halving the power does not lead to a perceived halving of intensity).

    A more scientific test would probably be to use a light meter, and something a little more sensitive than a Kill-a-Watt.

    EDIT: and welcome to Physics Forums!
  4. Jan 21, 2013 #3
    Hello Calvin : Welcome as well - AND thanks for a detailed and thought out post!

    I have no idea why you are seeing this behavior - I am stumped. There are some details about how the LED lamps typically operate - for example they are typically swithched on and of - much faster then you can detect with your eyes, this yields "apparent" brightness, so it's current flow is not really continuous - this low level, pulsed current may be interfering with how the kil-a-watt operates - just a wild guess.... I am thinking that it may take a lot of effort to figure this out - than you want to put into it - but a puzzling issue to be sure.
  5. Jan 21, 2013 #4


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    If you want to look more into this then you should use a 'better behaved' load. A resistance with an ammeter and voltmeter would be best. They should sort out your 'magic' effect.
    Or you could do a YouTube video of it and sell it all to some punter. :wink:
  6. Jan 21, 2013 #5


    User Avatar
    Science Advisor
    Homework Helper

    I assume you mean "plugged into a mains supply"?

    Most likely, when the mains supply is on, the UPS simply passes the mains direct to its output, so the Kill-o-Watt doesn't "know" the UPS was even there, and it works as it was designed to.

    But especially if it has an almost dead battery, the UPS is probably not producing output that looks like a 120V sine wave at 60Hz. Whatever it is producing is good enough to light your LED lamp, but it's not what the Kill-o-Watt was designed to measure.

    The quickest way to see (literally!) what is going on would be look at the UPS outout with an oscilloscope.
  7. Jan 27, 2013 #6
    You mention it is a led lamp? So maybe it has a converter with poor power correction in it, presenting a highly non-sinusoidal loads to the ups giving load surges every half cycle on the peak? If the meter has any significant inductance it might smooth out the load surges and make life a lot easier for the ups and its battery. Put a clip-on current probe on the circuit and see how the current waveform is affected by having the meter in series.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook