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.
I just mounted a GE microwave with a vent and it told me to mount at a minimum of 21 above the stove top. Comment from contributor P: I am just into this very problem with a client. The above stove cabinet was ordered for a standard vent hood over an electric stove. They then decided on a gas range. That didn't cause a problem. However, they then decided on a microwave unit above a gas range, and the microwave manufacturer, Panasonic, stated that the unit could be within a minimum of 13 5/8 from the countertop for an electric stove, and 16 for a gas range. Ah ha-now we have a problem!