3/1? Dont you mean 1/3?
It is useful. I have 3 UPS's in my house. Two on my desktop PC (one for the pc and one for the monitor. Needs to be like this because if its running on battery and you turn the monitor on, POOF! PC resets due to the surge

) The other UPS is on my server.
300VA - server - 15-20 mins
650VA - desktop - 20-30 mins
500VA - monitor - 20-30 mins
Ive got a 12v battery and inverter if I need it to provide another hour of power for the server or pc.