The general rule of thumb I was told when I went to get my new unit was to add up the Amp draw (listed on the backs of monitors, computers, ect...) of everything you plan on actualy plugging into the Battery outlets on the unit and multiply that by the voltage in your house (usualy 110 for most houses). This number you've figured out should be the target you use when you go shopping, just be sure that whatever you get has a rating higher than your calculated figure.
I was informed that under purchasing will result in your burning out the battery inside the Battery-backup in a matter of months instead of the years they designed it for.
Again, this is all based on what I was told, I honestly don't have any way to proove or dis-proove any of it. I decided that $100 was a safer bet than having nothing at all.
Hatchling Dragon