GS does not sell the Prius 12 volt battery in the US so it is hard to get
information about it. However, poking around techinfo.toyoto.com I found some!

The 2004 to 2009 ampere-hours rating for the battery is 28 without Smart Entry
System or Navigation system and 36 in with Smart Entry System or Navigation
System. This was under New Car Features – Appendix. However, for 2005 the
battery was listed as having 38 ampere-hour capacity without Smart Key or
Navigation and the usual 36 with the systems. Since it makes no sense to have a
bigger battery in cars without Smart Entry System this is probably a typo.

For the 2010 it looks like they dropped the 28 ampere hour battery and the only
one offered is the 36 ampere hour battery. It appears to be the same GS battery
and in the preproduction car I could see the GS label but I did not see one on
the production car I looked at.

The ampere hour rating for the Optima D51 replacement alternative is 38, which
is higher. This battery has tightly wrapped lead plates in a spiral design so it
has extreme vibration resistance and uses almost pure lead as the lead does not
have to be strengthened. This results in a longer lasting battery with less
voltage drop as it discharges and less voltage drop in cold weather. In fact
several users have told me they have gotten 10 years of use from their Optima
batteries. I have also been told that Optima batteries work great in Alaska!

Right now Optima is offering a $25 rebate on the battery. This makes the Optima
battery and the eLearnAid installation kit (with detailed installation
instructions) cheaper than the GS battery sold by Toyota. For more information
go to: http://www.elearnaid.com/12vo1topraub.html