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.
If you only want to run a TV or a laptop you may be able to get by with a much smaller inverter. Some small 400 Watt inverters plug into a cigarette lighter outlet and do a fine job. A good pure-sine inverter of this size will run your TV, DVD and Satellite. Or it will power a laptop or two and a printer. You could also install a small inverter permanently, say 400 or 500 Watts, to run home entertainment.