The thing is... these battery backup devices are designed for computers, monitors, printers, and yes, even keyboards and recording gear. But they are NOT designed to provide sufficient power to run a power amp, simply low load electronic devices. You actually CAN buy specific ones for very high capacity and drain, but they are very expensive and heavy.

To be honest, I've gone through innumerable brown-outs, complete power losses, lightning 'flickers' and the like, and I have never lost a power amp that way. They are, on the whole, pretty robust pieces of gear. Computers, and keyboards, OTOH, are far more susceptible to damage from this. So, when playing in less than optimal situations, I have a UPS that I put the keyboard on, but NOT the power amps.

The danger is that, if the power out is NOT just a flicker, but a complete loss of power, unless you shut down VERY quickly, the power amp will rapidly drain the battery, and you will STILL brown out the gear, including (and far more likely) browning out your keyboard or computer rig...

I live in Florida, capitol state of lightning.... By all means, protect your keyboards and delicate gear, but you will be unlikely to lose a power amp, in comparison