[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