You can download the owner's manual for any system you're considering from support.dell.com; the specifications page will list the weight.

Thanks for that, do you know if the published weight includes the power supply ?

If you go to the product page, they list all the weights of the laptop, excluding the power brick unit. The power brick can't weigh more than one pound, probably just 1/2 pound. And to comment on your concern for portability, usually if you are being very portable you'd want just the laptop with a long battery life, and you'd just pack the power brick somewhere else while you run around to use the laptop.