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.
See 3 ELECTRICAL GROUNDING INSTRUCTIONS on page 2. 1 MOUNTING SPACE This Microwave Oven/Hood requires a mounting space on awall as shown in Figure 1. It is designed to be used with standard12-inch wall cabinets.If the space between the wall cabinets is 36 or 42 inches, a FillerPanel Kit can be used to fill the gap.