I was worried about having it ready on time, plus my smoker had burned 2 kg of charcoal per hour on first burns (subsequently modified) so I took the 2 hours per lb upper estimate for a 12 lb pork shoulder x 2kg per hour and bought 7, 7kg bags of lump to be safe. On a 15 hour cook I actually used 11kg of charcoal, half a bag of hardwood logs and a couple of pounds of oak chunks - it seems that once the breeze blocks have some heat in them you need a smaller fire to maintain temperature.