I'm trying to verify whether a 10W aquarium heater is failing. It has a safety fuse that renders it permanently inoperable if it overheats by running unsubmerged, which is suspected in this case. My Kill-a-watt reports .1A, 12VA, 120V, but 0W. I thought I understood that positive VA or positive current and voltage should be inconsistent with 0 watts. What am I misunderstanding?


  1. I can measure an LED lamp at 1 watt and a filter pump at 5 watts, so it's not that 10 watts is below measurement threshold -- and I wouldn't expect different thresholds for VA vs the corresponding watts.
  2. It turns out the measurements with the heater plugged in are the same as with nothing plugged in. So I'm convinced the heater is shot, but what is the meaning of the Kill-a-watt showing 12VA but 0 watts when nothing is plugged in? Shouldn't 12VA and .1A at 120V correspond to roughly 10 watts? .1A seems suspiciously high too for just running the Kill-a-watt, and i would expect them to have "tare"d out the Kill-a-watt's own draw.
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    \$\begingroup\$ Sounds like your meter is partially damaged in some way that's causing it to falsely read reactive power with no load. If you aquarium heater is a simple switch and coil, you can probably test it (unpowered!!!) with an ohm meter. \$\endgroup\$ Commented Dec 2, 2020 at 3:17
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    \$\begingroup\$ You can have 12 VA and 0 W if your load is entirely inductive or capacitive. \$\endgroup\$
    – winny
    Commented Dec 2, 2020 at 8:57

1 Answer 1

  • Rounding error on the current reading
  • Your Kill-a-Watt is defective
  • Your Kill-a-Watt doesn't read down to 10W
  • You're feeding a reactive load (transformer, coil, or capacitor) that needs excitation current but doesn't actually consume power (this is a thing with AC)

I'd try it on a known-working 10W load, like a lamp with an LED light in it, or -- if you have one -- a working heater. If it can't read something that you know is consuming 10W, then it's borked.

To understand the last point, above, AC is different from DC, in that the RMS voltage times the RMS current can be nonzero, while the average power is zero. This can happen if the voltage and current are perfectly out of phase with each other. The instantaneous power may flow to the load for half a cycle of the AC wave, but it'll flow back again the next half-cycle.

See the picture for how this can happen:

enter image description here

  • \$\begingroup\$ thanks! i updated the Q with measurements of some devices under 10W, though i don't have a known load, i'm not worried that the kill-a-watt itself is malfunctioning or can't measure single-digit wattages. i'm confused how VA can be above threshold while registering 0 watts. \$\endgroup\$ Commented Dec 2, 2020 at 2:54
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    \$\begingroup\$ I've added an explanation on that. Reactive power is the reason that people talk about VA at all -- if it was all real power, it'd all just be called "watts". \$\endgroup\$
    – TimWescott
    Commented Dec 2, 2020 at 3:51
  • \$\begingroup\$ that makes sense, but with nothing plugged in, or a simple heater, why would the load be so reactive? and isn't the .1A suspicious when nothing is plugged in? \$\endgroup\$ Commented Dec 2, 2020 at 17:02
  • \$\begingroup\$ I missed that the first time around -- that's where "your Kill-a-Watt is defective or poorly designed" comes in. You'd expect some error; if the thing is measuring 0.1A and it's designed for a 10A full range then that's only 1% error -- that's not great, but it's not horrible, either, especially if it's not reading as real power. \$\endgroup\$
    – TimWescott
    Commented Dec 2, 2020 at 17:08
  • \$\begingroup\$ would you expect no load or a simple heater to be so reactive? \$\endgroup\$ Commented Dec 2, 2020 at 17:11

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