We have several different battery backups in place for our shop. Each design workstation has a battery backup unit with a total Watt supply approximately equal to the power supply unit in that computer. Since the power supply units are actually a little more power than the system really uses, this means you will get a little more life expectancy than is rated on the box hopefully. The only things we run off the actual battery backup is the computer tower and monitor(s) for that workstation, as other things like your phone, cell phone charger, whatever doesn't need to be drawing power away from your computer for those few minutes while on battery. We have tested the battery backups before while the computers were under full load, and while the battery backups only offer about ten minutes of power, it's more than enough to get your project saved and shut down your computer properly.
To get a battery backup big enough to run a large digital printer is VERY expensive and is way more expensive than efficient for us. A Mimaki JV5 printer has a rated power usage of "3,600 Voltamps or below" which means purchasing a battery backup of sustaining 3,600 VA. These are the type of battery backup used for sustaining a whole rackmount server, and can cost over $1,000.00 easily.
If you are using a smaller, less power-hungry printer, you may be able to find a large battery backup capable of handling the digital printer and being cheap enough to be a feasible option, but will still cost you at LEAST $200.00 for a good APC battery backup capable of running the smaller line of digital printers.