AFAIK yes there's a very large difference in power consumption. The hard drive and processors for laptops are designed to take less power, and a laptop is a 'closed system' so there's very little extra power headroom built in, i.e. a desktop computer might have a 300W power supply and it only really needs 250, but a laptop will have exactly the right amount and no more. I forget the typical total wattages for a laptop, but I'm pretty sure it's around or under 50W. Also, desktop monitors take up a large amount of energy as well. My 21" takes up between 100W and 160W. You oughtta feel the heat off of this thing!
From
this site: a typical laptop has maximum power consumption of 15 watts.
I don't know how old that is though -- 15W seem awfully low to me.