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.
Long story short: To install an over-the-range microwave oven, you need to have enough space between wall cabinets flanking the range and between the cooktop and ceiling. There should be a cabinet or shelf above the spot where the microwave will be installed so that you can fasten the top of the microwave to it. Most OTR microwaves are just under 30 in. wide; heights vary from about 10 to 18 inches and depths vary from 12 to about 18 inches.