To determine the appropriate level of hybrid bond issuance for TenneT Holding B.V., we analyze the provided financial data and industry risk factors:

1.  **Capital Intensity and Financial Position:** TenneT is a transmission system operator (TSO) with massive capital requirements. Property, Plant, and Equipment grew from €23.8 billion to €26.8 billion in 2022. Operating cash flows from investing activities were a significant outflow of €4.3 billion. The company is experiencing operating losses (-€976 million) and has substantial borrowings (long-term: €19 billion; short-term: €0.7 billion).
2.  **Hybrid Rationale:** TenneT already has €2.125 billion in hybrid capital. Given the high-growth, asset-heavy nature of the TSO sector and the aggressive investment requirements needed for energy transition infrastructure, maintaining balance sheet strength is critical.
3.  **Leverage and Rating Considerations:** With significant ongoing capex and negative operating income, TenneT faces pressure on its credit metrics. Hybrid bonds provide "equity-like" features that allow the company to manage its leverage ratios without diluting the ordinary shareholders (the Dutch State).
4.  **Market Conditions:** Interest rates rose significantly in 2022 (e.g., 10Y swap rate went from 0.053% to 1.927%), making the cost of traditional debt higher. The incremental cost of hybrid capital is now closer to traditional senior debt, reducing the "cost penalty" often associated with hybrids.
5.  **Assessment:** Given the "very high" capex intensity and the structural need to preserve an investment-grade rating while balancing massive infrastructure spending, TenneT fits the criteria for significant hybrid utilization. While the 15% cap is the theoretical maximum, TenneT's existing hybrid base and the need for fiscal discipline suggest an approach that supports significant leverage optimization without maxing out the structural limit, particularly considering the annual issuance constraint of €3 billion. 

Given the high capex intensity, the need for significant leverage support to protect credit ratings in a high-interest rate environment, and the scale of the company's investment program, an issuance level of 11.25% is recommended to balance leverage pressure with cost implications.

<answer>11.25%</answer>