The company already carries a substantial amount of hybrid financing (≈ 4.2 bn EUR out of ≈ 7.9 bn EUR total debt, i.e. roughly 53 % of debt). While hybrid instruments can be counted as equity for rating purposes, they are generally more expensive than senior debt, especially in the current environment where investment‑grade corporate yields are around 2.3 % for senior and around 2.5 % for sub‑ordinated paper, and hybrid issuance typically demands a further premium (≈ 3 %+). 

Acciona’s leverage (net debt/EBITDA ≈ 2.7x) and interest coverage (≈ 5x) are moderate, suggesting it can comfortably meet obligations with a lower proportion of costly hybrid debt. Reducing reliance on hybrid bonds would lower the weighted average cost of capital, preserve financial flexibility, and better align financing with the rising rate environment, while still retaining a meaningful hybrid component for rating support.

Therefore, a modest hybrid share of about one‑quarter of the capital structure strikes an optimal balance between cost efficiency and financial flexibility.

<answer>25%</answer>