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.
There are compact microwaves, medium capacity microwaves, and large capacity microwaves. Compact units are usually less than 18 inches wide, 14 inches deep, and 12 inches tall and their capacity is less than one cubic foot. The wattage of such units ranges from 500 to 1000 watts.