Give us some more info on your set up and we can help you out. Temps, how much charcoal are you putting in your fire box, are you using a charcoal basket etc.?
I don't use charcoal because it seems to burn soo fast I always use lump charcoal aka lump wood or lump wood and charcoal briquettes mixed.
I've got a strait-up no-mod SNP. I start with probably a couple chim fulls of Charcoal and add another another approx every 1 and a half hrs. I tried lump one time and couldn't keep the heat up (it was fairly cold and windy outside) and switched to kingsford mid smoke ( I know the purists are cringing and gagging). Maybe it was just a crap brand.
I used about 20 lbs yesterday for 10 hours of cooking time in my Char Griller offset. That was about what I expected. I'd love to use less, but offsets do use lots. I'm thinking about getting something smaller for most smokes that don't have as much meat as what I did yesterday. Maybe then I could use the Royal Oak with better success.
Also if you check out this link you will find a lot of good information on the kind of fuel you are using. Depending on what brand some burn longer/hotter/more or less ash etc. http://www.nakedwhiz.com/lump.htm
Yea I use lump charcoal aka lump wood and some times I will throw in some charcoal briquettes. When I say I don't use charcoal I guess I was thinking of the stuff you get that has the lighter fluid type stuff added (match light). Probably not calling it the right thing but hopefully you know what I am talking about.
lol... ok now I get it. I was scratching my head a bit on that one as well.
Drheerdt - horizontal offsets do go through a lot of fuel, especialy when you have cold ambient temperatures. I never bothered to fire up my old moded Char-griller unless temperature was at least above 40-45°. That is one of the reasons I switched to a WSM, it really doesn't care about cold very much.