I went from a desktop to a laptop simply because of power resources. A desktop now pulls more power simply because certain hardware needs more power.
If you own your own home and pay bills laptops are definatly one power saving feature of a home over desktops and with 9 hour + batteries coming into laptops they are superb devices!!
I will never go back to a desktop ever.
Surely if you used the same components (cpu/gfx/ram etc) in a desktop as you did a laptop, it would save considerably more power than the laptop due to not having the rather inefficient battery power, instead using a 80+% efficient psu.
You seem to be comparing a low spec laptop to a high spec desktop, in which case of course the desktop will use more power.