Debian and Ubuntu power management

Ubuntu makes some changes that in Debian are left to the user. However the biggest thing is likely to be that Ubuntu has a newer kernel and other software - there's been a lot of work in reducing power consumption recently. What version of Debian are you using, stable or testing/unstable? PowerTOP can be used to see what's causing your CPU to wake up (and hence use energy). That site also has a list of other things that can be tuned to save power.