T400 switchable graphics and power consumption

[pointyhat]

I have a T400 with switchable Intel/ATI graphics that I need to chuck CentOS on. CentOS doesn't support switchable graphics and the user doesn't need the ATI card's performance.

If I select integrated graphics in the BIOS rather than switchable graphics, does it completely power down the ATI core so it doesn't consume any power and doesn't appear on the bus?

[pointyhat]

Actually solved this by testing it. Stuffed my Fluke inline with the PSU (using a spare power socket bodged up) and pulled out the battery so it was running on mains only. Idle amperage is the same if you use the switchable graphics in integrated mode or disable it in the BIOS and force integrated mode. However if you kick the ATI in on switchable mode, the idle amperage increases (not by much but enough to be conclusive)

[ReCreate]

Indeed, the physical hardware is powered off. I have noticed a significant gain in battery life when I switch to integrated graphics. Usually 2-4 x as many hours as otherwise. (1 hour vs 4 hours)