Once you get the temp up inside the pit, you only need as enough "fuel" to counter the cooling effect of the pit as air goes through it. Doesn't take much.
Keep in mind, that hard wood burns hotter in general than charcoal or something - you can keep a small-medium sized charcoal smoker in bbq temp range with only 8-10 coals lit at a time. "one little log" is a hell of a lot bigger and produces more heat than 8-10 briquettes.
Cat Daddy's BBQ : 22.5 WSM / Weber OTG 22.5 Cajun Bandit / Smokey Joe Gold