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.
Small or compact microwaves range from 600 to 800 watts, while family-sized microwaves range from 850 to 1,650 watts. Small microwaves are energy efficient, but large microwaves typically cook food quickly. It is important to know the wattage of a microwave to correctly follow food preparation guidelines.