I don't know anything about the Kill A Wattmeter but I have heard of resistor problems before. Perhaps they have realised the problem and fitted bigger resistors.
I assume that anything that claims to be a wattmeter does in fact measure true watts. In which case I would expect it to measure rms current and volts.
If it is as described then it will measure accurately but it will be up to you to determine whether the answer is what you want.
The trick with msw inverters is that they use the same peak ac volts as true sine wave, this keeps items that rectify to dc happy so your radios, tv's and most electronic things work ok. They then change the on/off ratio of the square wave so that the rms value is the same as a sine. This means about a 50% on cycle. Things that rely on heat then work correctly such as incandescent lamps and heaters.
A true wattmeter will measure the rms current and voltage and the product will be watts so for lights and heaters the thing will be accurate. For any other form of load the thing will still measure the watts accurately but some appliances will not take the same wattage as on a pure sine. Motors may have high losses and take more current, things like microwaves often take less watts and have to run longer on msw for the same heating. Other loads can be very variable. Compact fluorescent rectify to dc and work fine, ballast type fluorescents often work badly, hum and don't run at the correct wattage.
In all these cases a true wattmeter will give the correct answer in the watts you use but it may be well off the figure some appliances may take on pure sine.
Flux
