Well you have to calculate their power usage to figure it out. I didn't honestly. I had 1 1000 va bb run 1 CPU, 3 19" LDD's, on skype talking to traders, running Ninja Trader, Sierra Charts and I got a whopping 9 minutes before the battery went dead! I was expecting 20-30 minutes. You just have to test it out after calculating and purchasing. No I run 2 1000 va bb's. cable modem, router, and 1 LCD and the rest of the stuff on the other. I got my bb's at Costco for $100 each.