What is value stacking, exactly?

There is no single, universally agreed definition of value stacking. Different organisations use the term to describe anything from "bidding into several markets" to "combining any revenue stream available to an asset." For this article, we use a specific working definition:

Value stacking is the practice of optimising the total economic value a flexible asset can generate across multiple markets, services and time periods, taking into account the technical, contractual, operational and market-related constraints that limit how much of that value can actually be captured at any given moment.

That definition matters because it draws a distinction between four related but different concepts:

• Revenue stacking simply adds up gross income from different markets or services, treating each euro earned as equivalent, regardless of the cost or opportunity forgone to earn it.

• Value stacking looks at net economic value: revenue weighed against degradation, opportunity cost, balancing risk and the value of capacity kept in reserve for a more valuable use later.

• Portfolio optimisation operates one level up: instead of optimising a single asset across several markets, a Balance Responsible Party (BRP) or aggregator optimises the net position of many assets together, sometimes deliberately leaving one asset idle because another in the portfolio is more valuable to activate at that moment.

• Market participation is simply the act of being active in a given market. An asset can participate in several markets without truly value stacking, if that participation is not coordinated around a clear view of where it creates the most value at each point in time.

These boundaries are not perfectly clean. Portfolio optimisation is, in effect, the same value-maximising logic applied across many assets instead of one, so the line between it and value stacking is partly a matter of scale rather than a difference in kind. We keep them separate here because the constraints involved differ in practice: value stacking is mostly about a single asset's technical and contractual limits, while portfolio optimisation adds the question of priority between different assets serving different purposes at the same time.

Why value stacking is becoming more important

Three developments make value stacking an increasingly important strategic consideration for Belgian asset owners.

First, price volatility is rising. According to Elia's own year-end market data, negative day-ahead prices occurred during 6.1% of the time in 2025, up from 4.6% in 2024 — an increase of about a third year-on-year — driven mainly by high solar and wind output combined with low demand on holidays and weekends [2]. An asset that only ever does one thing, such as charging on a fixed schedule or running at a constant load, is poorly placed to respond to a price curve that increasingly resembles a saw-tooth rather than a flat line.

Second, Elia's own Adequacy and Flexibility Study 2026-2036, published in June 2025, states plainly that flexibility needs to accelerate: with the right regulatory conditions in place, such as real-time pricing and digital tools, Elia estimates that smart flexibility could save the Belgian system €350-500 million per year in system costs by 2036 [3]. That need is being translated into market access: balancing products such as FCR, aFRR and mFRR have gradually opened up to smaller, aggregated and decentralised assets [4], and the CRM has contracted battery storage capacity in its 2025-2026 delivery-year auction alongside traditional generation [5].

Third, local grid constraints have started to matter alongside system-wide balance. Fluvius is explicit that today's issue is anticipated congestion, not an actual shortage: current capacity is sufficient, but the volume of new and upgraded connection requests from industrial users, data centres and battery projects is rising fast enough that some regions could face real constraints if nothing changes. In zones where this is expected, Fluvius offers a temporary flexible connection ("Fall-Back Flex") to businesses requesting 100 kVA or more: the connection normally delivers the full requested power, but Fluvius can temporarily reduce it during specific peak-risk moments to protect grid stability [6]. For an asset owner in one of those zones, flexibility is therefore not only a way to earn additional revenue; accepting some curtailment risk can be a practical condition for getting connected sooner rather than waiting years for a grid reinforcement.

Together, these developments push flexible assets away from a single, fixed role and towards a more dynamic one — exactly the terrain where value stacking becomes relevant.

From single-use to multi-value: two examples

The two examples below are illustrative constructs that reflect realistic Belgian market mechanisms. They are not based on a specific project or verified revenue figures.

Example 1: a battery energy storage system (BESS). A grid-connected battery does not have to serve one purpose throughout the day. Depending on its state of charge, technical availability and the markets it is prequalified for, its capacity could, in principle, be allocated differently at different moments:

• Part of its capacity could be reserved for FCR, a product that requires an automatic, proportional response to frequency deviations within seconds and is needed on a near-continuous basis rather than at specific times of day.

• Around midday, when solar generation is highest and local grid loading is at its peak, the same battery could shift towards absorbing local excess generation, reducing congestion at its connection point rather than exporting into an already saturated part of the grid.

• In the evening, as prices rise with demand, the battery could be discharged for arbitrage, or used by its BRP to help balance the wider portfolio rather than being dispatched purely on its own economics.

No single one of these applications alone would necessarily justify the investment. Combined intelligently, they can improve the overall business case. But the "intelligently" is doing a lot of work in that sentence, and we return to why in the next section.

Example 2: industrial demand response. A cold storage facility or an EV charging depot has thermal or operational buffers that allow some consumption to shift in time without affecting the underlying service. That flexibility could, again illustratively, serve several purposes at once: offering explicit balancing capacity to a BSP or aggregator when the price is attractive, shifting consumption away from the facility's own monthly peak to reduce network cost exposure, and respecting a DSO's dynamic capacity limit in a congested zone to avoid curtailment. Here, the "value" being stacked is not only additional revenue — part of it is cost avoidance, and part of it is the risk reduction that comes from avoiding a curtailment event that would otherwise disrupt operations.

The Belgian market: what is actually possible

A theoretical list of markets an asset could serve is not the same as a list of markets it can realistically combine. For any given Belgian asset, four separate questions need to be answered: Is it technically possible, given the asset's response time, ramp rate, state-of-charge headroom and metering setup? Is it economically interesting once opportunity cost and degradation are priced in? Is it allowed under current rules and contracts — some combinations are constrained by prequalification conditions, "mutually exclusive bid" structures in CRM and balancing auctions, or exclusivity clauses with an aggregator or supplier? And is it operationally feasible to forecast, bid, dispatch and settle across several markets and counterparties without conflicts?

Belgium currently offers a reasonably broad, if still evolving, set of building blocks. FCR, aFRR and mFRR — Belgium's three balancing products, which differ mainly in how fast they respond to a grid imbalance — have progressively opened up to smaller, aggregated and decentralised assets [4]. The CRM works differently: it is a security-of-supply mechanism that compensates capacity holders facing a so-called "missing money" problem, where a capacity's investment cost is not covered by what it can earn on the energy market, with the aim of keeping existing capacity on the market and attracting new capacity to avoid future adequacy shortfalls [7]. Because it is designed to close that funding gap rather than to replace energy market income, a CRM capacity payment can still be received alongside revenue from the energy markets and from balancing and redispatching services — though not without limits: Elia's own CRM rules require capacity holders to pay back part of their remuneration if the day-ahead price exceeds a pre-set strike price, and CRM capacity cannot at the same time receive other variable subsidies such as green certificates or CHP certificates.

Congestion services are a third building block, and one worth separating out on its own, since it is easy to mention "redispatching" in passing without explaining what it actually is — and, unlike most of the products above, it is not a new addition. Elia has long used redispatching to manage physical congestion on the transmission grid: altering generation or consumption patterns to change power flows and relieve a constraint, as defined under EU Regulation 543/2013 [8]. Redispatching is not reserved for conventional generators — Elia's iCAROS programme has progressively formalised how flexible capacity, including storage, can be coordinated for this purpose, applying the same principles to energy storage as to generating units [9]. At distribution level, Fluvius is building a more recent, comparable set of tools: its "Fall-Back Flex" product lets businesses in a congestion zone accept a flexible rather than a fixed connection in exchange for access to a local flexibility market [6, 10], and Fluvius has published a broader roadmap of additional flexibility products still under development for managing local congestion across its network [11]. None of this changes the combinability logic described above: congestion services sit alongside FCR, aFRR, mFRR and the CRM as another layer an asset's capacity can, in principle, serve — subject to the same technical, contractual and operational limits that run through this article.

At the same time, some of these building blocks are shifting. Standalone battery parks connected to the high-voltage grid have benefited from an exemption on certain network tariffs since 2017, originally intended to offset the fact that storage is metered, and therefore charged, both when it consumes and when it injects electricity [12]. As part of its 2028-2031 transmission tariff methodology, CREG has indicated it no longer considers this exemption justified, citing more than 25 GW of battery projects currently under study by Elia — roughly five times the capacity Elia has identified as necessary by 2030 — and has proposed a phased withdrawal for storage installations without a signed connection contract by April 2026 [13]. As this is a regulatory proposal still working through CREG's tariff methodology process rather than a long-settled rule, the exact timeline and scope are worth reconfirming against CREG's own published decision before publication. It is nonetheless a useful illustration of a broader point: the profitability of a given value stack is not fixed, but depends on a regulatory framework that itself responds to how much capacity the market has already built.

A related mechanism applies at household and SME level in Flanders. Since 2023, part of the Fluvius network tariff has been based on a connection's peak quarter-hourly power (the capaciteitstarief) rather than on volume alone [14]. Various consumer-facing sources cite this component as accounting for a significant share of the total network cost for smaller connections, with figures of up to roughly 30-40% appearing in secondary reporting; we have not been able to confirm that range directly against a Fluvius or VREG publication, so it should be treated as indicative rather than exact. The direction is clear regardless: it is a real incentive to flatten peaks even where no explicit market participation is involved, and a reminder that not every layer of value stacking is about selling a service — some of it is about avoiding a cost already built into the tariff structure.

Strategic implications for asset owners and energy companies

For organisations investing in flexible assets, three things follow from this. The business case for a flexible asset should be built around a realistic mix of value streams from the start, not added later — adding it later is possible, but it usually means fitting extra metering, communication and control equipment that would have been cheaper to include from day one.

Value stacking is also not a one-time design decision. Because market rules, tariff structures and price patterns keep evolving — in Belgium's case sometimes at short notice, as the battery tariff exemption shows — the systems and contracts that support a value stack need to stay flexible themselves, including aggregator or supplier contracts, which should avoid unnecessary exclusivity where possible. And organisations managing a portfolio of flexible assets should think about optimisation at the portfolio level from the start, rather than building single-asset business cases in isolation and combining them afterwards. The two are not the same exercise, and the second one tends to leave value on the table.