I need to analyze whether REN should utilize hybrid bonds and to what extent relative to S&P's 15% cap on equity credit for hybrids.

Let me consider the key factors:

**1. Capital Structure Analysis:**
- Total Equity (2023-01-01): €1,517,534,000
- Long-term Borrowings: €1,695,362,000
- Current Borrowings: €638,944,000
- Total Debt: ~€2,334,306,000
- Total Adjusted Capital (Debt + Equity): ~€3,851,840,000
- S&P's 15% cap: ~€577,776,000 maximum equity credit from hybrids

**2. Leverage Assessment:**
- Debt-to-Equity ratio: ~1.54x
- This is moderately leveraged, typical for regulated utilities/infrastructure companies
- REN is a regulated energy network company (electricity and gas transmission)

**3. Interest Rate Environment:**
- Swap rates have risen dramatically from negative territory in 2020-2021 to positive ~1.7-1.9% in 2022
- Credit spreads (iBoxx) have also widened
- This makes new debt issuance more expensive
- Hybrid bonds would carry even higher coupons than senior debt

**4. Business Profile:**
- REN is a regulated utility with stable, predictable cash flows from concession assets
- Revenue is primarily from rendering services (regulated tariffs)
- Strong operating cash flows: €613M in FY2022
- The company has significant intangible assets (concession rights): €4.08B

**5. Considerations for Hybrid Usage:**
- **Pro:** Regulated utilities often use hybrids to optimize their credit ratings while maintaining investment capacity. The equity credit helps improve leverage metrics at rating agencies.
- **Pro:** REN has substantial capital expenditure needs (concession asset investments ~€200M/year)
- **Pro:** The company's stable regulated cash flows support hybrid coupon payments
- **Pro:** Given rising rates, equity credit from hybrids could be valuable to maintain rating without issuing expensive pure equity
- **Con:** Hybrid coupons are higher than senior debt
- **Con:** The company already has moderate leverage for a regulated utility

**6. Typical Utility Practice:**
Regulated European utilities frequently use hybrid bonds to the full extent of rating agency equity credit caps. Companies like EDF, Enel, Iberdrola, and similar peers commonly issue hybrids. REN, as a Portuguese regulated transmission operator, fits this profile well.

**7. Financial Flexibility:**
- The company pays regular dividends (~€102-144M)
- Cash flows from operations are strong but capex needs are ongoing
- The rising rate environment creates incentive to optimize the capital structure

Given that REN is a stable, regulated utility with predictable cash flows, moderate leverage, and ongoing capital needs in a rising rate environment, hybrid bonds would be a natural tool for capital structure optimization. European regulated utilities typically utilize this instrument significantly. However, the company's leverage isn't extremely stretched, and they seem to manage well with conventional debt. 

For a regulated utility of this nature, I would expect moderate-to-significant utilization of the hybrid cap. Most regulated utilities use 50-100% of the available capacity. Given REN's moderate leverage and stable profile, but not extreme need for equity credit, I'd estimate around 50% utilization as reasonable - enough to benefit from the equity credit while not over-relying on the more expensive hybrid instrument.

<answer>50%</answer>