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 minimum size inverter that will run small microwaves and coffee makers is 1500W. Match the Inverter to your Battery Bank, and to the appliances that you will be running. An energy audit will tell you what you really need. If you haven't done one take a look here.