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.
Approximate Running Wattage (what is this?) 240V required? Refrigerator or Freezer (Energy Star) 1200. 132-192 . Microwave Oven 650 watts. 1000. 1000 . 800 watts. 1300. 1300 . 1000 watts. 1500. 1500 . Incandescent Lights. as indicated on bulb (i.e. 60W) as indicated on bulb (i.e 60W) Furnace Fan, gas or fuel oil . 1/8 Horsepower. 500. 300. Y. 1/6 Horsepower. 750. 500. Y