As a motorcyclist I have one. It's 38W and keeps you toasty. Mine is a Keis which plugs into the bike wiring harness, but is also available with a portable battery pack..
I have seen different ones on sites, some very cheap, which I would keep away frome Guzzilazz, a motorcycle mate has an expensive one, and it's great.
Myself, I just have a fur lined ( imitation) one that is toasty.