I bought this watt meter from eBay. The case is a nice extruded aluminium case, but it is unusable for low currents. For example it shows 30 mA for a 100 mA load, tested with a passive resistor:

And still about 28% error for 1 A, tested with a passive resistor as well:

I guess for higher currents it gets better.

Internally it uses a shunt in the ground path, which is called low-side current measurement. This might be a problem if you have other things connected to ground to the device you are measuring:

It works down to about 4.7 V. But it should be possible to desolder the red wires from the board and solder a wire for an external power supply to the PCB.

The display is soldered to the board, so I can't see details of the circuit:

Conclusion: if you want to measure high currents above 1 A and don't care about accuracy, you can use this. Otherwise don't buy it.