Originally Posted By: ConRAD
Are you sure that the model you've linked is ok ? from its instructions manual, chapter#2 I read:
- The measurement range concerning the fundamental frequency is 40 to 70 Hz.
- Voltage/Current: Sinusoidal signal (THD < 0.1%)
- Power: Sinusoidal signal, PF = 1, Frequency 50 Hz, Measuring range 10.00 W...999.9 W

- at 20 km/h you might expect 32-36 Hz
- expected THD is well above 20%
- Power measuring range has a minimum starting from 10W

It is using a internal 2kHz sampling and calculates based on the instantaneous values. It is not the high end option, but it did work reasonably well here (PWM for heating, 100Hz, 25 - 500W). It does not really need a low THD (Current (THD = 40%), typical error 0,2%) and it does not stop working completely outside the usual 40 - 70Hz window. Just don't trust it at 200Hz. But i never tested it at lower power ... crazy

The better option (Fluke) is officially limited to the same frequency rage. But that one does work at higher frequencies.

Digital 2 channel oscilloscope is the better option, but more complex in use. I used one to check the cheaper wattmeter. You don't really need the wattmeter if a oscilloscope is available ...