You haven't provided much detail what you are asking about so I'll throw out a couple of comments. Energy efficiency can be described as:
Reactive power versus the actual kWh you are consuming (your power factor). Utilities can charge you a premium for non unity power factors. Capacitor banks are typically used to correct power factors but you would need to look at your contract and actual power factor to see if it is economical to fix this.
Electrical use in the plant. I would think you need to do an audit, identify where you are using electricity and see where you could have savings. Compressed air leaks, electrically driven pumps with high dPs across control valves, etc all will increase energy consumption. Items like higher efficiency lighting, turning off lighting, etc.
As far as an 'index', you would need to supply us what type of plant, the size, location, etc and perhaps someone can give you some guidelines what your energy consumption should be.