I believe, and it has been my experience, that one of the largest challenges we face today is the issue of capacity. Maybe you have read or heard that capacity has to do either with the amount of things to want do and the amount of time you need to do them. Well, that is an appropriate way to look at it, but there is also the thing about efficiency or how fast can you do things as well as about effectiveness or how precise are you on completing what you set yourself to complete since the beginning.
You can be neither efficient nor effective if you lack the energy to do it. I thought this Infographic from Mind Tools is pretty simple and a good invitation to explore the aspect of developing your energy.