To determine the appropriate extent to which REN's capital structure should rely on hybrid bonds, we need to evaluate the company's financial profile, industry characteristics, and the strategic role of hybrid capital.

1. **Industry Characteristics and Regulatory Framework:**
REN operates as a regulated electricity and gas network operator in Portugal. Utilities in this sector are characterized by highly predictable, stable cash flows, monopolistic positions, and heavy capital expenditure requirements (reflected in REN's massive Intangible Assets, largely concession assets, and Property, Plant, and Equipment). This stability allows utilities to sustain much higher leverage than typical corporations. Regulators often evaluate capital structures based on notional equity, where hybrid bonds are typically treated as 50% equity and 50% debt due to their perpetual nature and deferrable coupons.

2. **Current Capital Structure:**
As of the end of 2022, REN's total equity is approximately €1.52 billion, while total liabilities stand at €4.93 billion, resulting in a Debt-to-Equity ratio of about 3.25x. This is relatively high but standard for European regulated utilities, which frequently rely on hybrid bonds to optimize their weighted average cost of capital (WACC) without diluting common shareholders. 

3. **Role and Optimal Weight of Hybrid Bonds:**
Hybrid bonds serve as a sliver of the capital structure designed to credit-optimize the balance sheet. Because they are deeply subordinated and perpetual, they cushion senior debt holders, earning partial equity credit from rating agencies (usually 50% floor) and regulators. However, due to their high cost (typically issuing a premium over vanilla senior debt) and the risk of stepping up or coupon deferral, they are not meant to replace either core senior debt or common equity. For a utility of REN's size (total capitalization around €6.45 billion), a hybrid bond stack typically ranges from €500 million to €1 billion. This corresponds to roughly 10% to 15% of the total capitalization, or approximately 25% of the total equity base. 

Relying on 0% would miss the clear WACC optimization and rating-agency management opportunity available to a stable utility. Relying on 50%, 75%, or 100% of the equity base (or total capital) in hybrids would be excessively aggressive, leading to unsustainable coupon pressures, loss of financial flexibility, and would violate the standard regulatory notional equity thresholds (which cap the eligible hybrid portion around 25% to 30% of the regulatory asset base). Therefore, a 25% reliance relative to the equity/equity-like target is the standard, optimal threshold for this sector.

<answer>25%</answer>