As Za1n said, I've done the same thing with my Fluke setup and found similar results... One thing to keep in mind, is that in general the higher the support watts it can monitor, the lower the resolution.  You're not going to be able to measure mW with something that measures thousands of watts - or more specifically, you can but it's going to cost you a bunch of cash.  and as tadeus1 points out, you also need to be able to calibrate it for whatever the voltage source you're using it - and make sure the device can measure the voltage source, in case there are dips, etc.

I used to use a Kill-a-watt ages ago, then I switched to a Watts Up Pro UC (supports 240v), and lately I've fallen in love with these:

http://amzn.to/2DwPJDX

They're cheap as hell, can handle input voltages from 110v up to 240v - and give realtime data.  Better still, you can take a known load, like a space heater or something similar, and calibrate them so they work even better.  The only PIA with these is you have to wire on the connector yourself, but it's not that hard and also gives you the option to use C13/C14 connectors, traditional 3 plugs, etc.  I've also modified all the ones I used to use one of the open-source projects, and then modified that to be more specific to my purposes - all in all an awesome option.

I also have one of these installed at home and at all our mines:

http://amzn.to/2DL0vKo

Worth every penny - you can configure it with all you your utilities billing nonsense and it's accurate within a few dollars of what my bill is every month.  But as I said above, the larger the monitored load, the lower the resolution - so where the first one is accurate within a couple watts, this one is accurate within tens of watts or more depending on the size of the install (like on our 1600a mines).