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.
Answer Wiki. A typical home microwave transformer has two secondary windings. One winding provides 3.1 to 3.2 volts, while the high voltage winding provides between 1800 - 2800 volts (average ~2200 volts). The low voltage output is used to light the filament in the microwave-generating vacuum tube (called a magnetron).