Imagine that we could find a way to tap into a tree's conducting tissues, and sap sugars from it. Not all of them, leave enough to keep the plant alive and healthy. 

There are two options. We could refine this sugar, and sell it, or we could try to go another (giant) step of the way.

Attempt to get electrical energy from sugar directly, somehow. I don't know a way of doing this that not include cellular respiration, If that is not possible, move on.  

Devise a chamber that consumes the sugar, through artificial celullar respiration. Instead of producing ATP, you would gather the electrons generated in the process.

The motivation to tap into plants isn't to be mean to them, it is actually to find a sustainable way to conserve forests. Rather than to uproot forests and replace them with crops, you would actively work along with the trees. It would be in your best interest then to keep the trees healthy.

A point of doubt is that should this idea of messing with plants metabolism hold, it could be more profitable to sap into sugar cane than native species. In that case, this idea just died in terms of helping the environment.