To determine the extent to which IBERDROLA SA should utilize hybrid bonds, we need to evaluate its capital structure, cash flow dynamics, capex intensity, and the comparative cost of issuing new hybrids against its existing cost of debt.

1. **Capital Structure and Adjusted Leverage Metrics**:
At the end of 2022, Iberdrola reported total equity of approximately €58.1 billion and total financial liabilities (current and non-current) of €69.3 billion. The Total Adjusted Capital (TAC) thus stands at roughly €127.4 billion. 
Iberdrola generated a robust €10.4 billion in operating cash flows against €6.27 billion in capital expenditures (PPE purchases). Because operating cash flows comfortably cover these capital outlays, the company avoids the "significant leverage pressure" described in higher hybrid utilization buckets (11.25% or 15%). However, with €25 billion in current financial liabilities indicating moderate refinancing needs, its rating headroom (BBB+ / A- equivalent) remains moderately constrained by its steady investment pace and dividend payouts.

2. **Cost of Debt Comparison**:
Iberdrola’s gross finance costs were €3.04 billion in 2022 on an average debt base of about €64-69 billion, implying an average cost of debt of around 4.4% - 4.7%. 
Looking at the 2022 market data, the 5Y and 7Y swap curves averaged 1.73% to 1.81%. Combined with a Sub-Sen Delta (Non-Financial IG) of roughly 2.30% and an estimated issuer spread, a newly issued hybrid bond would price near 4.5% - 5.0%. Thus, the cost of issuing a hybrid bond will only *slightly increase* the company's current overall cost of debt. 

3. **Hybrid Bond Capacity**:
Iberdrola is already an active issuer in the hybrid market, paying €169 million in subordinated perpetual interest in 2022. This implies an existing hybrid base of approximately €8.25 billion (or around 6.5% of TAC). By allowing for up to €3 billion in new hybrid issuances over the next 18 months—primarily to optimize its leverage metrics while navigating its moderate refinancing queue—Iberdrola's hybrid stock would sit naturally within the 7.5% cap target.

**Conclusion**:
Iberdrola fits the profile of a strong, regulated/unregulated utility that uses hybrids as an optimization tool rather than a lifeline. It exhibits moderate refinancing needs, faces moderately constrained rating headroom due to steady growth investments, and would only face a slight increase in its cost of capital when issuing a hybrid. 

<answer>7.5%</answer>