I have a used Kill-A-Watt power meter and want to see how accurate it is. In my "conventional" middle-class USA apartment, am I likely to have any electricity-consuming devices that I could use to test this?

(I am guessing that some appliances have pretty variable power consumption which would make it hard to test, but that maybe others reliably use some wattage that is listed on their label. If that's true, then maybe I could use one or more appliances from that second group to test?)

  • I would use a multimeter and buy a few resistors for the low end, but that involves playing with mains voltages and even in Japan or the US that's not for everyone. One option is renting a proper power meter and comparing the outputs. But that's likely to cost more than buying a cheap one. FWIW my PowerMate meter gives the same results as my $20 cheap one down to about 5W, at which point the cheap one guesses and is often within a factor of two of the correct result (it's not consistent either, so it's not very useful for sub-5W loads).
    – Móż
    Jan 19 '21 at 21:55

According to the manual, the Kill-A-Watt is calibrated to accuracy within 2%, with 0.5% being typical. In contrast, any conventional electrical load which you might have in your house won't be calibrated at all. It will have some maximum current limit, but beyond that, even for the same product, the actual load might vary by as much as 10% -- there's simply no incentive for manufacturers to "calibrate" those loads.

If you think the Kill-A-Watt is notably inaccurate (which is unlikely without some other visible symptoms), you could compare it to resistive loads, such as a space heater, water heater, hair dryer, or toaster. These will have constant loads and should roughly match the value stated on the label.

For example, if you had this 1875 W hair dryer, you could connect it through the Kill A Watt, and run it at the highest setting for 6 minutes. The display should read roughly 0.1875 kWh.

If you wanted to take this further and you have access to your read your utility electric meter and to your electric panel, you could try this:

  1. Turn off all the breakers and wait for the meter to "settle." Then take a kWh reading.
  2. Find a breaker with only outlets and lights. Often there will be a panel outlet -- a single breaker feeding only one outlet, right next to the panel. Electricians often set this up first to be able to have power while doing other work.
  3. Connect the Kill A Watt and some large load (toaster, heater, etc) on that outlet and run it for a while.
  4. Take a kWh reading from the meter, subtract the first reading, and compare the result to the value on the Kill A Watt.

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