APC UPS Question for 3.2 Mac Pro

I can tell you I have the APC BACK-UPS RS 1300VA LCD 120V

Plugged into the battery backup side is:

- Linksys 4 port router
- Cable modem
- OWC Mercury Elite 500GB external
- 23" Apple Cinema Display
- Mac Pro 2.66 with 3 internal drives

With everything on the APC shows I have 17 minutes of runtime.
With everything on except the display, it shows 21 minutes of runtime.

Just an fyi.

-Kevin