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 microwave size challenge. My KD has given us specs for our over the range microwave they are 30wX15-16Hx12-14D. The Width is fairly standard, limited quantities on the 15-16 Height. I can't find anything 12-14 depth.