My a/c units(3), watermaker, dive compressor, refrigerator, etc have 110v 15 amp plugs on each power cord with dedicated GFI receptacles to match. This was done to make it easy to remove the units if it should become necessary, and the telltale lights on the GFI receptacles are a nice confirmation that you have power if something doesn't want to run. A hidden advantage is that I can plug each item into my Kill-A-Watt and see how much juice it consumes. Of course the microwave, electric kettle, toaster etc have plugs anyway, so I can compile a fairly complete list of how much each electrical item on the boat consumes and then load, but not overload, my 6.5Kw generator.
I would not recommend running the heavier appliances through a Kill-A-Watt for any length of time as they tend to get hot (according to some reviews), but a couple of minutes while taking a reading seems fine.
It should be born in mind that things like air-conditioning compressors take a lot more juice when starting than when running. Unfortunately the Kill-A-Watt does not seem to react fast enough to measure the start-up current.
One of these short extension cords can make the display much easier to read.