Revenue Stacking for BESS Projects

Battery storage systems in Germany can participate in multiple markets simultaneously, significantly improving project economics. This principle — revenue stacking — is central to project valuation, but comes with regulatory and technical constraints that must be carefully accounted for.

Available Revenue Streams

Grid-connected BESS projects in Germany can generally access the following revenue streams:

• FCR (Frequency Containment Reserve): Primary control reserve, symmetric, 30-minute products since 2024. Remunerated via capacity price (€/MW/week).
• aFRR (automatic Frequency Restoration Reserve): Secondary control reserve, prequalification required, capacity and energy prices. Higher revenue potential, but also higher activation probability.
• Day-Ahead Arbitrage: Buy at low prices (overnight, high renewable feed-in), sell at high prices (evening peak). Volatile margins, strongly dependent on spread development.
• Intraday Arbitrage: Shorter positions, higher trading frequency, lower predictability.
• Local Grid Services: Reactive power compensation, voltage support — project-specific, rarely scalable.

Constraints When Combining Revenue Streams

Not all combinations can be realised simultaneously. The most important restrictions:

Physical limits: A battery can only use its State of Charge (SoC) once. Holding FCR requires keeping a symmetric SoC buffer free (typically 50% ± reserve). This reduces the capacity available for arbitrage.

Prequalification requirements: FCR and aFRR have strict technical minimum requirements (response time, measurement accuracy, communication). Failure during the prequalification period leads to exclusion.

Cross-border bidding zone rules: FCR is procured at European level (PICASSO, TERRE). Simultaneous bids across multiple markets require precise bid management, as overbidding incurs penalties.

Optimisation Approach

A realistic revenue stacking model accounts for the SoC path across the entire dispatch horizon. FCR revenues are modelled as a fixed capacity product, while remaining capacity is optimised for arbitrage. The choice of planning horizon is crucial: hourly day-ahead optimisation systematically underestimates the value of intraday flexibility.

Market Development and Spread Compression

As more BESS projects enter the market, FCR capacity prices decline structurally. Models that extrapolate historical FCR revenues significantly overestimate future cash flows. Robust project valuations must therefore include scenario ranges for declining arbitrage spreads and capacity prices.