Decision-oriented indices for quantifying stability and supply-side diversity in decarbonizing power systems

Achieving net-zero electricity requires not only reducing average emissions but also managing hidden stability risks arising from low synchronous inertia and poorly diversified generation portfolios. This paper introduces two transparent, data-driven indicators based on open hourly generation data: the Carbon Inertia Index (CII), which measures the demand-weighted share of supply that structurally requires synchronous, inertia-providing resources, and the Flexibility Entropy Index (FEI), which quantifies supply-side portfolio diversification using a Shannon-entropy formulation. Applied to 2024 ENTSO-E data, the indices reveal distinct transition archetypes across Europe, ranging from inertia-rich but concentrated systems such as France to inertia-poor yet diversified systems such as Spain, and show that the continent spends almost twice as many cumulative country-hours in low-CII regimes as in inertia-rich ones. Embedding CII and FEI as linear constraints in PyPSA-based capacity expansion models for Spain and Poland demonstrates how stability and diversification requirements reshape least-cost portfolios, increasing curtailment and, in some cases, limited load shedding while improving resilience signals. These results show that cost-optimal, emission-focused planning alone can hide risks linked to inertia scarcity and portfolio concentration, and that simple, reproducible indices can help planners and policymakers design power system transitions that are both cleaner and more robust. • Presents two easy-to-use measures: Carbon Inertia Index (CII) and Flexibility Entropy Index (FEI). • Measures how stable and flexible a power system is using open hourly generation data. • Shows clear differences in energy mix and stability across European countries. • Uses CII and FEI inside PyPSA models to test how stability goals affect system costs. • Helps planners and policymakers design cleaner and more reliable energy systems.