I find the "divide by ten" rule works fairly closely with inverter draws. If the 120v appliance pulls 900w, it is about 90 amps draw.
You can't go just by the watts label on the appliance, but it gives you some idea of what it will want. You can see on the inverter's watts read-out what it actually pulls.
Also a microwave pulls less from a MSW inverter than from a PSW inverter, but it also has lower cooking power. Some MSW inverters have a hard time with microwaves and other inductive loads. Pick a MSW inverter that is rated to run "motors" for best results if you don't go to PSW.