How can energy consumption be an indicator of per capita availability of capital?
In economics, capital refers to things that are used to produce other things. 

So, capital includes tangible things like a delivery truck, an office building, or a factory. These are things which often require energy to run.

Hence, if a developing country has low capital per head, then it probably also has few delivery trucks, office buildings, and factories. And hence, it probably also has lower energy consumption.