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.
Operating a Laser Printer with a Power Inverter. A laser printer generally requires a pure sine wave inverter capable of surging at least 6.5 times the maximum wattage rating of the printer. For instance, a laser printer rated at 500 watts requires an inverter with a surge rating of at least 3,250 watts.