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.
A power inverter converts DC power into conventional AC power which can run all kinds of household products such as: kitchen appliances, microwaves, power tools, TVs, radios, computers and more. You just connect the inverter to a battery, and plug your AC devices into the inverter and you've got power on the go.