I have a simple problem. When I am using Kubuntu 12.04 and doing some simple stuff like IRC, Chrome, Tor and Okular, my battery lasts 1-1.5 hour.
With Windows 7 and same kind of programs, it lasts about 3 or more hours. Is this normal?
If you have switchable graphics, then that is very likely to be the problem. As far as I know there is no linux support for switchable graphics cards. I recently learned that this may be about to change, but for the moment you're stuck.
The integrated graphics card on your mainboard uses much less energy than the dedicated one. Windows can switch from one graphics card to the other, greatly increasing battery life. Since Linux does not support this feature, if your BIOS is set to let the Operating System chose which graphics card you use, Linux is probably using the dedicated card, hence the loss of battery life.
As far as I know, the only solution would be to disable the NVIDIA card on the BIOS. The intel card is probably enough for most of your Linux needs. If you want to play a game, reboot, switch to OS select again in the BIOS and boot into Linux.
If you have switchable graphics (as probably is the case with a Dell XPS 17), look at bumblebee.
It will automatically disable the nVidia card when not needed (it's on by default); this should give you better battery life.