Anyone looking to get involved in doing this may like to look at a new app 
note from Microchip.

http://ww1.microchip.com/downloads/en/AppNotes/01279A.pdf which is for a 
1KVA (1.3KVA for 2 seconds) Offline UPS, capable of 220V/50hz or 110V/60hz 
operation.

Should be modifiable to do what the OP was wanting. 

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