Certification to ISO 50001 occurs after you have developed a complete EnMS and have preformed at least one Internal Audit. Then you select a registrar that is ANAB certified for ISO 50001.
At a minimum you complete aGap analysis. This is an optional step of assessment to take a look at any existing energy management system and compare it with ISO 50001 requirements. Then a Formal assessment occurs to ensure your preparedness an procedural control.