This dispute, brought by Luxembourg-based investor 9REN Holding S.à.r.l. against the Kingdom of Spain, arose from a series of significant alterations to Spain's renewable energy regulatory framework. The Claimant invested approximately €211 million in 2008 to develop eight solar photovoltaic (PV) plants, relying on the stable and guaranteed feed-in tariff (FIT) regime established under Spain's Royal Decree (RD) 661/2007. 9REN argued that between 2010 and 2014, Spain enacted measures that systematically dismantled this regime, thereby frustrating the legitimate expectations upon which its investment was based. These measures included limiting remunerated operational hours, imposing new taxes, altering inflation indexation, and ultimately revoking the FIT system in favor of one based on a 'reasonable rate of return.' The Claimant asserted that these actions constituted breaches of the Fair and Equitable Treatment (FET) standard, the non-impairment provision, and the umbrella clause under Article 10(1) of the Energy Charter Treaty (ECT), as well as an unlawful expropriation under ECT Article 13. Spain contested the Tribunal's jurisdiction on several grounds, including the intra-EU nature of the dispute, which it argued, citing the CJEU's Achmea decision, precluded investor-state arbitration. Spain also invoked the ECT's denial of benefits clause, alleging 9REN was a mere 'letterbox' company, and challenged jurisdiction over claims related to a 7% tax (IVPEE) based on the ECT's tax carve-out. On the merits, Spain defended its actions as a legitimate exercise of its sovereign right to regulate in the public interest, particularly in response to a severe economic crisis and a ballooning 'tariff deficit.' It maintained that its legal framework had always been dynamic and that investors were only ever promised a 'reasonable rate of return.' The Arbitral Tribunal, composed of Ian Binnie (President), David R. Haigh, and V.V. Veeder, unanimously affirmed its jurisdiction. It rejected the intra-EU objection, finding the ECT's consent to arbitration to be clear and unconditional. It also dismissed the denial of benefits and corporate pyramid objections. While declining jurisdiction over the IVPEE tax as a standalone claim, it considered its economic effects in the overall assessment. On the merits, the Tribunal unanimously found Spain liable for breaching the FET standard under ECT Article 10(1). The Tribunal determined that RD 661/2007, through its explicit language and associated government representations, had created a specific and reasonable expectation of stability for the FIT regime applicable to seven of 9REN's eight plants. The subsequent radical changes frustrated these expectations. The Tribunal noted that the eighth plant, registered under a later decree (RD 1578/2008) which contained an explicit warning of future modifications, did not benefit from the same expectation of stability. The claims for expropriation and breach of the umbrella clause were dismissed. The Tribunal awarded the Claimant damages of €41.76 million, representing a 20% reduction from the amount claimed. This adjustment accounted for factors such as a reduced operational life for the plants (30 vs. 35 years), the exclusion of the eighth plant from the stability guarantee, and discounts for illiquidity and regulatory risk. The Tribunal ordered Spain to pay pre- and post-award interest, compounded annually, at a rate equivalent to the 5-year Spanish government bond yield. Spain was also ordered to bear the majority of the Claimant's legal costs, totaling US$4,814,570 and €562,458, in addition to the full costs of the arbitration proceedings amounting to US$299,908.16.