Has anybody had experience with either of these units? How they perform, accuracy wise, with small loads < 10W?
A lot of my work is in energy efficiency and I would like to have a decent instrument for measure the power consumption of AC devices. I have a few Kill-A-Watts and they are fine as far as they go, but they are pretty inaccurate. I have units that differ by more than 5%. Also, they are not very helpful for very small loads like the "vampire" loads of many standby power supplies that have relatively high crest factors.
A low-cost solution to your problem occurred to me. The Kill-a-watt has an internal shunt consisting of a short piece of manganin wire. I bought another Kill-a-watt at Fry's, removed the shunt and measured it. It's a 2 milliohm shunt. I then replaced that shunt with a .2 ohm, 2 watt 5% wirewound resistor, choosing the most accurate one from a bunch of them. Now the modified Kill-a-watt has 100 times the sensitivity. I was able to measure the no load draw of a modern wall wart at .077 watts. The maximum power measurable is now 18 watts. I compared the reading with my high sensitivity, low power factor Yokogawa wattmeter, and it was very close.
For the final accuracy check, I just set the modified Kill-a-watt to measure current with a small load, and compared that to the simultaneous current reading meaured with a DVM. The modified Kill-a-watt reads 2% low. A person could trim the .2 ohm shunt, or just add 2% to the reading. To trim, I would start with a .2 ohm resistor that measured high with full length leads, as measured with a 4 terminal ohmmeter, and then you could solder in the resistor with near full length leads, and then shorten them as a method of trimming. Or, solder in a .22 ohm resistor and solder in an additional resistor in parallel to trim.
You probably don't need exceptional accuracy for vampire loads, and this solution is well within your budget.