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.
But for those who are already struggling to free up their countertop space just to make way for their brand new ovens, Whirlpool WMC20005YD will make one of the ideal microwave oven consideration. At about 11-inches wide, it is by far smaller than the typical microwave design.