TenneT Holding B.V. should be advised to use hybrid bonds at the **maximum S&P equity-credit cap of 15% of total adjusted capital** over the next 18 months.

Key reasons:

- **Existing and recent hybrid usage is a strong positive signal.** TenneT already has **€2.125 billion of hybrid capital outstanding**, first issued in 2010, and it issued hybrids in 2021 or 2022. This indicates hybrids are an accepted and proven part of its capital structure, rather than a new or untested instrument.

- **Leverage pressure is extremely high.** The reported S&P metrics are very weak for an investment-grade regulated utility:
  - **Net debt / EBITDA: 83.85x**
  - **FFO / net debt: -1%**
  - Moody’s adjusted leverage trend: **deteriorating**
  
  These figures suggest very limited rating headroom and a strong need for capital structure support.

- **Large investment and funding needs.** TenneT is a regulated transmission system operator with a very large and growing asset base. Property, plant and equipment increased from **€23.8 billion to €26.8 billion**, while capex was very high at **€4.4 billion** in 2022. Investing cash flow was **-€4.3 billion**, and funding needs were met largely through debt, with **€7.3 billion of new borrowings** and net financing of about **€6.0 billion**.

- **Debt increased materially.** Long-term borrowings rose from **€12.4 billion to €19.0 billion**, a very large increase in one year. Although TenneT also received **€1.23 billion of shareholder capital contributions**, this was not enough to prevent deterioration in leverage.

- **Regulated utility profile supports hybrid use.** TenneT operates essential electricity transmission infrastructure and is state-owned by the Dutch State. This gives it a strong business risk profile relative to many sectors, but the scale of the energy transition capex program means financial risk is the binding constraint. Hybrid bonds are therefore especially useful to protect credit metrics while preserving access to debt markets.

- **Cost is less favorable than in 2020–2021 but still strategically justified.** Euro swap rates and corporate bond yields rose sharply in 2022, so hybrids would be more expensive than prior issuance. However, given TenneT’s severe leverage pressure and ongoing investment program, the rating and balance-sheet benefits outweigh the higher coupon cost.

- **Annual issuance capacity is sufficient.** The instruction allows up to **€3 billion of hybrid issuance per year**. Over 18 months, TenneT could plausibly issue enough additional hybrids to move closer to the 15% S&P equity-credit cap, especially given its very large adjusted capital base and existing €2.125 billion hybrid base.

Overall, TenneT has the characteristics associated with the **15%** category: transformational capex needs, material leverage deterioration, heavy reliance on external financing, constrained rating headroom, and an established track record of hybrid issuance.

<answer>15%</answer>