I need very small form factor UPS, the smaller the better. The only requirement is it must have USB management interface so I am able to shutdown a PC gracefully...

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Most of the size and weight of a classic UPS is the lead-acid gel-cell in it. That battery determines the maximum current and runtimes the UPS can deliver, So you can't spec a UPS until you know your power and runtime constraints. Further, the circuitry to turn the battery's DC into (slightly) clean mains AC is inherently wasteful, twice so since all modern electronics internally require DC voltages, meaning there's a second wasteful conversion back to DC voltage(s) in the PC's power supply.

My current solution for keeping low-power computers alive without mains power is to power them directly from one or more lithium cells which are continuously charged from mains power. If you're running something like a RaspberryPi, it's easy to simply watch the battery voltage, when it drops below ... say 3.5 volts for a single cell, the Pi can trigger a shutdown. (Restarting after power returns has been left as an exercise for the student. :)

Have a look here: Power Supply comparison You're interested in the column labelled 'Interface', and you'll want devices with serial or USB interfaces if you're trying to connect a laptop or small desktop. The mini-box products come closest to a complete out-of-box solution for your task, but seem overpriced, especially since the cells ore not included. OTOH, perhaps their kit is exceptionally well made and worth the price.

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