I just had a bout with the energy part of HASS that I couldn’t make work until now. I realized the unit I had for my energy usage was “kwh” and not “kWh”, as listed in the docs. The energy component immediately recognized my entities as being able to provide consumption data.
I propose that an alert or something is produced, if the unit used on a sensor entity does not make sense. It turns out it’s case sensitive - please tell me (the user) when my sensor isn’t useful at all … 
I imagine this has larger consequences, for example normalization to common units between sensors etc…