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.
HOW MANY WATTS DO YOU NEED? Appliance : Watts: Radio: 0.5: 100 W. Light Blub: 100: Blender: 200: Window Fan: 250: Color TV: 300: Clothes Dryer (Gas) 400: Dish Washer: 400: Computer: 400: Freezer: 500: Refrigirator/Freezer: 600: Bread Maker: 600: Hot Tube Pump: 750: Microwave: 800: Coffee Maker: 850: Iron: 1000: Waffle Iron: 1100: Electric Frying Pan: 1150: Toaster : 1150: Toaster Oven: 1200: Electric Range One Element : 1200