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.
Decide what type of microwave oven you want. Countertop models are the most conventional kind of microwave. They range in size from 18 inches wide and 11 inches high (45.7 cm wide and 27.9 cm high) to 24 inches wide and 14 inches high (61 cm wide to 35.6 cm high).