If you need to charge the ThinkPad X1 Carbon or Lenovo Yoga Or ThinkPad Helix in a car or on an airplane (it can be also used as a normal power adapter), you can use the Lenovo AC/DC Combo power adapter (90 Watts), and the Lenovo slim power tip. This setup is both compact (weighing around 0.35 kg for the entire setup, or 0.23 kg for the DC part), and energy efficient (conversion efficiency of over 95%).

Lenovo 90 W Ultraslim AC_DC Combo Adapter

ThinkPad Slim Power Tip for X1 Carbon

Furthermore, the Lenovo AC/DC Power Adapter tip is easily swapped over to suit the different IBM and Lenovo laptops (i.e. IBM ThinkPad, Lenovo IdeaPad).