[Powertop] eth0 using 11 W when not in use

Using version 1.97 from Ubuntu repos (Xubuntu 12.04 LTS). Powertop reports ~15 W discharge on my Acer Aspire 7736Z, with estimated battery life of about 2.5 hours. This is normal battery life for its current full state of 39.0 Wh. (Battery is a 48.6 Wh, but has lost about 20% in the 2 years since bought new). Under Overview and Device stats, Network interface: eth0 (tg3) is consistently at the top, with a power estimate of 11.4 W. I never use a wired connection on my laptop, and there is no way in the BIOS or via keyboard Fn to disable it. The wireless interface, wlan0 (ath9k), typically reports <1 W, sometimes as high as 6 W. Is Powertop accurate in reporting my wired network interface at 11.4 W? Any troubleshooting tips for checking this or even disabling? I have found very little info on Powertop and disabling eth0 in my Googling, so thought I'd try the mailing list. Side question: I don't know what to do under Tunables. What does toggling between Bad and Good do? I tried looking for documentation but could find none. There is no man page... Thanks, Tim