To determine the appropriate extent to which Iberdrola's capital structure should rely on hybrid bonds, we must evaluate the company's current financial structure, the nature of hybrid bonds, and the macroeconomic interest rate environment.

1. **Current Capital Structure and Use of Hybrids**: 
Looking at the 2022 balance sheet, Iberdrola's total equity and liabilities amount to €154.667 billion. The equity attributable to owners of the parent is €41.119 billion, and noncontrolling interests are €16.995 billion, bringing total equity to €58.114 billion. 
The company already utilizes hybrid-like instruments, as seen in "Instrumentos De Capital Con Caracteristicas De Pasivo Financiero No Corriente" (€576 million) and "Corriente" (€87 million). Furthermore, the cash flow statement shows "Emision De Obligaciones Perpetuas Subordinadas" (Issuance of Subordinated Perpetual Bonds) of €2.74 billion in the previous year, with accrued interest payments recognized directly in equity ("Devengo De Intereses De Obligaciones Perpetuas Subordinadas"). Thus, Iberdrola already uses perpetual subordinated debt—a classic form of hybrid bonds—as a supplementary capital tool, representing a single-digit percentage of its total capitalization.

2. **Nature of Hybrid Bonds for Utilities**: 
Hybrid bonds (such as perpetual subordinated debt) possess both debt-like features (coupon payments) and equity-like features (perpetuality, subordination, coupon deferral options). For capital-intensive, regulated utilities like Iberdrola, they are highly advantageous for credit rating agencies (like S&P and Moody's) which often treat them as 50% equity under certain thresholds. This allows the company to raise capital without diluting existing shareholders or over-leveraging with senior debt, effectively optimizing the weighted average cost of capital (WACC). However, relying on them too heavily (e.g., 50% or more) would subvert their purpose; rating agencies would cap their equity credit, and excessive subordinated perpetual debt would drastically increase the firm's cost of capital and refinancing risk.

3. **Macroeconomic Environment**: 
The provided swap curves show a rapid shift from negative interest rates in 2020-2021 to significantly positive rates in 2022 (e.g., 5Y Swap rising to 1.726%). Meanwhile, credit spreads (iBoxx Euro Corp Bond and Sub-Sen Delta) have also widened in 2022. The rising rate environment makes the issuance of perpetual hybrids much more expensive than it was during the low-rate era. Therefore, over-relying on hybrids in the current cycle would be cost-prohibitive.

4. **Conclusion on Extent**: 
Hybrid bonds should serve as a "sweetener" or marginal capital instrument rather than a core funding source. They are optimal at a moderate supplementary level—typically around a quarter of the equity structure for utilities maximizing their "AA" or "A" rating band without triggering rating downgrades due to over-reliance on hybrid capital. 0% would leave rating optimization benefits on the table, while 50%, 75%, or 100% would be excessively expensive and structurally unsound, failing to meet senior lender safety covenants. Therefore, a 25% reliance relative to the equity/hybrid tranche is the standard, prudent ceiling for utility hybrid capital optimization.

<answer>25%</answer>