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.
Just connect the inverter to a battery, and plug your AC devices into the inverter and you've got portable power whenever and wherever you need it. The inverter draws its power from a 12V or 24V battery (preferably deep-cycle), or several batteries wired in parallel. power inverter changes direct current (DC) power from a battery, usually 12V or 24V, into conventional mains alternating current (AC) power at 230V. This means that you can use one to operate all kinds of devices ... electric lights, kitchen appliances, power tools, TVs, radios, computers, to name just a few.