I am planning to buy Resonate RouterUPS CRU12V Power Backup for WiFi Router. The documentation says this model is compatible with all 12V, <0.9A powered WiFi or WiFi/ADSL routers. Will it support airport express?

compatible with all 12V, <0.9A powered WiFi or WiFi/ADSL routers. Will it support airport express?

No it won't.

The AirPort Express is a 100-240V router, so If you want a backup for the Express, you will need a device that supplies 110 Volts in the U.S., Canada, or Mexico and 220-240 Volts in most of the rest of the world.

Quite possibly, check the ouput of the airport express wall adapter. If its output is 12v and less than 1amp it should work.

Both the older and current models of the Airport Express require direct connection to mains electricity - there is no low voltage input and the Power Backup device in question will not work. The OP would need a mains UIPS (uninterruptable power supply); there are many models available and they can maintain your Mac as well for long enough to provide a graceful shutdown in the event of a power cut.

check the ouput of the airport express wall adapter.

The AirPort Express does not have a wall adapter or outboard power supply. It plugs directly into 110 Volt or 220 Volts depending on where you live in the world.