looloo,
I've always thought of it from the view that while the size of the community is directly involved with social complexity, without surplus energy it is difficult to have large groups of people to habitating together. Since social complexity has strong correlation with community size, and community size has a strong dependence on available surplus energy, therefore social complexity is in some way levels dependent on surplus energy. I don't expect there is a perfect correlation between social complexity and surplus energy (some societies may use their resources more efficiently than others and some societies with surplus energy may make the conscious decision to stay small), but it makes sense that the two are related. As to seeing the evidence of the relationship, just think of all the things that surplus energy helps you do (feeds you, transports you, gives heat & light & sophisticated shelter, etc), things that would otherwise occupy the majority of the available hours of the day to procure. Surplus energy takes care of many of the basics for us at far less expense of time and effort, which allows us to be more specialized in our careers and skillsets, which allows for the complexity and diversity of human activities in society. I think Chris Martenson did a good job of explaining it in the Crash Course, but we've all become so accustomed to the idea of bountiful energy being around that it can be hard to grasp it on a personal level without having any experiences in low surplus energy environments.
- Nick

In all this series this is the one "key concept" that I am struggling to grasp . I always thought that social compexity was derived from the size of a community group , as in a twenty person community will not support say; a barber shop whereas a thousand person community might, I do not understand how surplus energy makes a contribution unless in some way large communities are not feasable without surplus energy but this is not evident to me.