I've been using a Kerio Auto Air Adaptor for years to power my Dell notebook off my DC rig:
http://www.kerio.com.tw/products.php?id=17
It's been great and the guys at Kerio are amazing. When I first bought the unit, it didn't have the Dell tip; so I contacted them, fully expecting to pay for a few for spares and they sent them to me from Singapore free of charge - wouldn't even let me pay for postage. Same thing happened a few years later when I purchased a new Dell.
Anyhow, definitely no complaints, I've had it operating for over 5 years in sub-zero to 45C+; but I was just looking at the specs and noted this:
"Over 90% power efficiency"
How far over 90% I'm not sure - but it's got me thinking about how much juice I may be losing. But perhaps this sort of conversion is about the best you can get for these types of notebook adaptors. Any thoughts?