Shelley was a key member of a close circle of visionary poets and writers that included Lord Byron; Leigh Hunt; Thomas Love Peacock; and his own second wife, Mary Shelley, the author of Frankenstein.
If the 900 watts means the input power, then 1000 w inverter would be ok. If the microwave produces 900 w of cooking power, you will need a bit more, maybe 1200-1400 w if you don't know what the input power rating of the microwave is.
Medium capacity microwaves come with a width of 20 inches or more and with capacities of 1 1.5 cubic feet. Such type usually has auto-cook features that are not common in compact units. Large capacity microwaves, on the other hand, come with a plethora of cook features plus more accurate temperature controls.