Currently, custom containers can only store item stacks and fuel progress. It would be nice to add energy (or more general resource) storage as well. The idea is:

• Users (server owners) can add energy types to custom containers. Each energy type will get a name, maximum amount, and initial amount.
• Container recipes may add, consume, or require energy. (The user controls which recipe adds, consumes, or requires which type(s) of energy and how much.)
• Energy indicator slots can be used to display how much energy of a certain type the container has left.