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.
Inverter/chargers have a unique feature that allows automatic operation when wired into the RV's electrical system. HOW IT WORKS. The INVERTER supplies 120VAC to selected plugs, microwave oven or TV/VCR sets 24 hours a day while dry camping.