**Reasoning**

1. **Current hybrid position** – TenneT already has €2.125 bn of hybrid capital, which represents about **7.4 %** of total adjusted capital (equity + adjusted debt ≈ €28.9 bn). This leaves headroom to increase hybrids up to the S&P cap of **15 %** (≈ €4.33 bn), i.e. about €2.2 bn of additional hybrid capacity.

2. **Funding environment** – The company is in a high‑capex phase (≈ €4.4 bn in 2022) driven by grid expansion and offshore‑wind connections. Cash flow from operations (≈ €1.2 bn) does not cover these investments, so external financing is essential. Large refinancing needs also exist: long‑term borrowings of €19 bn mature over the next few years.

3. **Leverage and rating pressure** – With a debt‑to‑equity ratio of ~2.8×, negative net income (‑€879 m loss in 2022) and a weakly positive FFO/debt ratio (~6 %), the company faces significant leverage pressure. Maintaining an investment‑grade rating will require optimisation of the capital structure, and hybrid instruments provide equity‑like credit treatment without diluting equity.

4. **Cost considerations** – Hybrid bonds would be priced above senior debt (swap curve + credit spread) but still below the cost of equity. The impact on the overall cost of debt would be marginal at the 11.25 % level, making it an efficient financing tool.

5. **Guideline alignment** – The “11.25 %” tier matches TenneT’s profile:
   * **High capex intensity** (large, ongoing investment programme).
   * **Significant leverage pressure** (high debt, negative earnings).
   * **Investment‑grade preservation strongly dependent on capital‑structure optimisation**.
   * The cost of a hybrid issuance at this level would have only a marginal effect on the current cost of debt, satisfying the guideline condition.

6. **Maximum issuance limit** – The guideline caps annual hybrid issuance at €3 bn. To reach the 11.25 % target, TenneT would need to issue roughly **€1.1 bn** of new hybrid bonds, comfortably within the annual limit.

**Conclusion**

Given the company’s high capital‑spending requirements, elevated leverage, need to preserve its rating, and available hybrid headroom, the optimal level of hybrid financing for the next 18 months is **11.25 % of total adjusted capital**.

<answer>11.25%</answer>