**Analysis**

1. **Capital structure snapshot (FY 2022)**  
   - Total equity (including non‑controlling interests): €13.8 bn  
   - Adjusted debt (borrowings + other financial liabilities + institutional‑partnership debt): ≈ €31.0 bn  
   - **Total adjusted capital** ≈ €44.8 bn  

2. **Funding picture for the next 18 months**  
   - **Refinancing:** €4.2 bn of current‑portion debt matures in 2023; additional maturities will arise in 2024.  
   - **Capex:** Cash‑out for PP&E & intangibles was €3.5 bn in 2022 and is expected to stay at a similar level as the group pushes forward with renewable‑energy build‑out.  
   - **Acquisitions:** Small deals (Sunseap, Central Térmica) are on the horizon, but the pipeline is not transformational.  
   - **Cash generation:** Operating cash flow ≈ €3.8 bn (2022), leaving limited internal funding after the high capex requirement.  

3. **Credit metrics**  
   - Net debt/EBITDA ≈ 4.5× (debt ≈ €25 bn, EBITDA ≈ €4.5 bn).  
   - FFO/Debt ≈ 15 % – acceptable for a mixed regulated/unregulated utility but leaving limited headroom.  
   - Debt‑to‑capital ratio ≈ 55 % (leveraged).  

4. **Rating considerations**  
   - EDP sits in the lower‑investment‑grade zone (likely BBB+/BBB).  
   - With capex intensity and a tight FFO/Debt cushion, the rating is vulnerable to a modest increase in leverage.  
   - Hybrid issuance would count as 50 % equity credit, reducing adjusted debt and improving leverage metrics, thereby preserving rating headroom.  

5. **Cost of hybrid vs. senior debt**  
   - Current market rates (5‑yr swap ≈ 1.7 %, iBoxx corporate average ≈ 1.1 %) imply a cost of senior debt of roughly 2‑3 % for a BBB+/A‑ issuer.  
   - Hybrid securities would likely price 100‑150 bps higher (≈ 3‑4 %). The incremental interest expense is manageable given the group’s EBITDA (~€4.5 bn).  

6. **Sizing the hybrid tranche**  
   - The S&P cap limits equity‑credit on hybrids to **15 % of total adjusted capital** → max ≈ €6.7 bn.  
   - Annual issuance ceiling is **€3 bn**, so over an 18‑month window the firm could issue up to €6 bn while staying within the per‑year limit.  
   - To meaningfully improve leverage (net‑debt/EBITDA from ~4.5× toward the 3.5‑4.0× range) and provide rating headroom for the large refinancing and capex programme, a **mid‑size hybrid tranche** is appropriate.  

   - **7.5 % of adjusted capital** → ~€3.4 bn (close to the annual ceiling and would only partially offset the €4.2 bn refinancing need).  
   - **11.25 % of adjusted capital** → ~€5.0 bn (covers the refinancing, adds a buffer for the capex pipeline, and materially lowers the leverage ratio).  
   - **15 %** → ~€6.7 bn (largest possible, but the added interest cost would be unnecessary given the funding gap can be satisfied with a smaller hybrid).  

   **Conclusion:** The optimal balance is **11.25 % of total adjusted capital**, which translates to roughly €5 bn of hybrid bonds (€3 bn in year 1 and €2 bn in year 2), staying inside the €3 bn/year issuance cap and providing the necessary rating support without over‑leveraging the cost of debt.

**Final answer**

<answer>11.25%</answer>