One thing I look at with these inverters in regards to capacity is the Fuse rating.
If a 12V inverter is rated to say 2000W and you see the thing has 2x 30A fuses which always seem to be on the DC side, You know the thing hasn't got a hope in hell of actually doing that output.
For 12V > 240V, I work on a 10X factor for input to Output Amps to watts. Time you take into account varying voltages on the input side, varying efficiency of the inverter and other factors, 10 is a good, safe, easy to work with number to calculate with.
IE, 100W out = 10A in.
So many of these inverters just don't add up. Like many things Chinese, The numbers advertised have absoloutley no relationship to fact or reality. I suggest Crunching the numbers and seeing if they add up to the speil given. With a lot of these things, you'll find that 1+1 Apparently = 14.
I have gone through a lot of these inverters and some are OK and some are just pure and utter garbage. As well as being woefully under rated, they won't handle the inrush current of a 100W light bulb let alone anything else regardless of their rating. Others drop out at 1/4 their rated load, I suspect through garbage internal wiring which won't flow the required current and others have bad heat dissipation problems.
I was lucky enough a couple of months back to win a very high quality Inverter on fleabay that was near new and went for about 1/20th of it's still current retail price. The difference between the chinese cheapies and this quality ( but still Chinese made) inverter is Night and day.
Yeah, the cheapies are adequate for some things and some I have had great service from some but others are just next to useless.
Generally the higher the rating, the greater chance of it being a load of Bullshit.
The fuse rating I found is a real easy tell tale as to what the units capability truly is and the honesty of the seller or manufacturers ratings.