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.
Quick Answer. Over-the-range microwaves come as small as 1.1 cubic feet, which translates into about 28 inches wide, 15 inches deep and 12.6 inches high. The smallest countertop microwaves are even smaller at 0.7 cubic feet. Keep Learning.