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.
The recommended inverter to run 1200 Watt continuous and 4800 Watt peak surge microwave is 2500/5000 Watt inverter or larger. In general, induction motors require an initial surge of power to start up (starting load or peak load).