The challenges posed by climate change, on the one hand, and energy shortages, on the other, are frequently in the headlines. Accelerating the global deployment of renewable energy is both essential and strategic for mitigating climate change and strengthening energy security here and there. Achieving this objective requires substantial financial and technological investments, which can be supported by both domestic legal frameworks and international arrangements. International Investment Agreements (IIAs), in particular, play a pivotal role in facilitating the inflow of foreign investment, especially in the renewable energy sector. Among their provisions, the standard of National Treatment (NT) is central to liberalising investment flows and fostering foreign participation. This article therefore examines how NT clauses can be more effectively designed to protect and promote foreign investments in renewable energy—an issue of considerable economic, legal, and academic significance. Through a detailed analysis of NT clauses in existing IIAs (lex lata) and by providing recommendations for their future clauses (lex ferenda), the article concludes that, despite some progress, most IIAs still adopt a superficial approach to the protection and promotion of renewable energy investments. In particular, the absence of pre-establishment NT clauses in most agreements significantly undermines their effectiveness. The article ultimately highlights the legal techniques that states should consider when drafting IIAs to attract and safeguard foreign investment in this sector.