Damages and reparation in energy related investment treaty arbitrations interpreting and applying rules of customary international law regarding state responsibility

International investment tribunals are frequently required to interpret and apply rules of Customary International Law (CIL) in investor-State disputes. This article examines how investor-State tribunals, in particular those constituted on the basis of the Energy Charter Treaty (ECT), have interpreted the CIL 'full reparation' standard regarding damages and reparation. By reference to ECT jurisprudence it is established that tribunals often utilize teleological interpretive tools to give content to this norm. Furthermore, some critical comments are made concerning the manner in which ECT tribunals subsequently apply the 'full reparation' standard. It is argued that the combination of the commonly adopted approaches to interpretation and application may explain why investors are occasionally capable of obtaining significant amounts of compensation in these public law disputes.