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.
Over the range microwaves require a minimum 66 mounting height from floor to top of the micro. The 66 from the floor dimension will allow approximately 13-16 between the cooktop and the bottom front of the microwave.