Its a good point - I think the language being used in productivity discussion is painfully informal and wooly.

I think energy in this context is often used to mean "enthusiasm" or "motivation". Its the energy that can be applied in a desired direction - a combination of actual gas in the tank as well as a panoply of psychological factors.

> You can borrow your energy almost ad infinitum to the moment that > you're going to experience burnout and heavy depression

Sama says something quite interesting here that indicates the psychological rather than the energy aspect of burn-out. He believes its impossible to burn-out if you are experiencing success. I think there is truth there, failure to progress and dis-satisfaction is a common feature in burnout cases. I suspect burnout is often a protection mechanism to get us to stop a behaviour rather than an actual state of energy depletion. Its a bit like the growing evidence for psychosomatic backpain occurring in response to psychological not physical factors.