How many watts does your plane produce? What does that even mean?! Watch this episode of Flite Test about watts, volts, amps, etc... and learn how to measure this information to improve your flying experience.
I am considering this watt meter, however, I cannot find any reviews where this unit was checked for accuracy and proper calibration when received factory fresh.
I have seen other reviews of meters that show most meters are of questionable quality and accuracy from the factory, even ones claiming 0.01 +- accuracy. Is it possible for these meters to go out of calibration sitting on the shelf or during shipping?
so . . . To keep this in perspective, we're talking about a ganged voltage/current meter with a fancy calculator for wattage, an integrator for mAh, and a few resistors tossed on to balance-drain a pack, in a hansom aluminum case . . . all for $20.
Accurate? To the stated precision? Maybe. Probably not. Close enough, though?
Drift? There's a chance of it, but it's all solid-state, so I'd suspect more of a chance of thermal drift if the device lacks compensation than from any other source . . . but by how much? probably not much at all.
At this price, professional gear with quality and conformance certifications is unrealistic. If you need the right number to a fine measurement, every time, then this device is not for you. If you're trying to build up a lab rig to precisely measure for a commercial or research purpose, this is not a place to cheap out.
If you're building a test-bench to double check your plane's power system against specs, or measure current draw for sizing, or to gather real-world motor measurements for yourself or other hobbyists, It should be just fine.
For "hobby" use, I wouldn't worry too much about those last few digits. Yes they may change, but if you measure a 32.1A one time and 32.5A another, will this change the sizing for your ESC? C-rating on your batteries? Whether Prop brand/size "A or B" is better suited to your needs? Whether your motor is Ok/border-line/in-danger? The difference is down in the noise -- The precision and accuracy may not match the significant figures given, but for these purposes, you need the rough number, not the precise number.
Calibration issues are likely to be not that important even for some more commercial type test applications when it comes to component sizing. Differences between static and dynamic thrust and power draw are likely to be much bigger then the accuracy issues a device like this is likely to experience. Bench tests really only test static values so you will need to leave enough margin in there to handle the the dynamic power draw that can be significantly higher in some cases. (specifically for ducted fans and high pitch props)
Even the high end expensive gear usually reports one more digit then it can accurately measure. This is so the user can get a feel for the noise characteristic and guess if need be. The safe thing is to always discard the last digit, sometimes two digits if it is cheap and measures to the thousandths. Like I already said, other requirements will cause you to need enough extra margin in your components that problems due to measurement errors shouldn't be a problem.