EMS, BESS and markets
A storage system is only worth what it decides. This page says what the control system arbitrages, what pays for that arbitrage, and what conditions every euro. Without quoting a single one.
The vocabulary, once and for all
The BESS, Battery Energy Storage System, is the complete system: the cells, the conversion, the cooling, the safety systems. That is what Stellabox delivers, under the Mercury name. The EMS, Energy Management System, is the supervision layer that decides when to charge and when to discharge. On the Mercury units it sits in the same cabinet: there is no extra enclosure to add, no integrator to bring in to link the two.
It is this control system that shaves, arbitrages and switches over, and it is what makes the site dispatchable by an aggregator. A battery without an EMS is just a reserve: it is the decision that creates the value.
The Mercury 418 carries predictive management compatible with aggregation. CESC manufacturer data.
What the control system does, second by second
Five steps running continuously. A setpoint is not fixed in the morning for the whole day: it is recalculated as consumption, generation and prices become clearer.
The power drawn by the site, the generation, the state of charge of the cabinets.
Consumption in the hours ahead, expected generation, announced prices.
Charge, hold, discharge. The setpoint is recalculated continuously, not once a day.
The conversion applies the setpoint, within the cabinet's power and temperature limits.
The gap between setpoint and reality is measured, and corrects the next decision.
Trading, without the folklore
One cabinet can serve several uses, which stack up. Sites in this sector quote percentage gains and payback periods here. We do not have those figures for the Mercury units, and an industrial buyer carries them into their calculation: so we will not write them.
What we can say, and what is worth more than an unverifiable figure: what each layer depends on, and who holds the answer. The base of the stack reads off your bill. The top requires a country, an aggregator and a qualification.
The site commits to raising or lowering its power on request, and that commitment is paid for.
The control system buys energy when it is cheap and releases it when it is expensive, on wholesale prices.
Charge during off-peak tariff hours, discharge during peak hours.
The midday solar surplus is stored, then released when the site consumes.
The battery covers the power overrun instead of the grid. The meter sees only smoothed consumption.
These layers do not add up freely. The same hour cannot be sold twice, and a flexibility commitment ties up a share of the capacity, which is then available for nothing else. That is the arbitrage the control system makes, and that is what is priced in the quote, on your load curve and your contract.
Country by country
The names change from one country to another, the families do not. Here is what to look for in your contract and with your grid operator.
| Famille | What it is | What it requires |
|---|---|---|
| Capacity pricing | The bill carries a share that depends on the power drawn, not only on the energy consumed. | Nothing beyond the current contract. It is the simplest mechanism to verify. |
| Hourly and seasonal prices | The price of energy changes by hour, by day or by season. | A contract that exposes those differences, and a load curve that lets you play them. |
| Wholesale market | Energy is bought and sold day-ahead, then within the day. | A supplier who passes those prices through, or an aggregator who carries the positions. |
| Frequency reserves | The grid operator pays for capacity held available to stabilise the frequency. | A technical qualification, a controlled response time, and capacity tied up. |
| Demand response and local flexibility | The site commits to reducing its consumption on signal, over announced periods. | An aggregator, a commitment contract, and metering compliant with the country's rules. |
| Capacity mechanism | Capacity available at times of system stress is paid for. | A certification, and rules that vary widely from one country to another. |
Families of mechanisms observed on European markets. Whether they are open to on-site storage, their rules and their thresholds depend on the country, sometimes on the region. Stellabox supplies the equipment and its control system: access to a mechanism is verified with your supplier, your aggregator and your grid operator.
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Families of mechanisms that pay for on-site storage.
Their names change from one country to another, the families do not. None is open everywhere: access is verified with your supplier and your grid operator.
Observed on European markets
The same Sunday, two hours
This is arbitrage, without the metaphor: the wholesale price of the same megawatt-hour, recorded twice on the same day. At 17:00 solar is still producing and demand has not returned; at 19:00 the evening peak arrives and solar has withdrawn. Between the two, the gap is a market's, not a brochure's.
What the control system draws from it depends on your contract, your country and your aggregator. We show the gap, which is public and verifiable. We deduce no revenue from it, which would not be.
The gridThe battery
Solar is still producing and demand has not come back. On half the markets a megawatt-hour is worth less than one euro, and in Spain it goes below zero: the grid pays to be relieved. This is the hour the control system buys.
Spot price in euros per megawatt-hour. Same scale on both sides, up to 280.
The site, then the aggregatorThe battery
Two hours later solar has withdrawn and the evening peak arrives. The same megawatt-hour is worth 152.98 euros in France, 213 in Germany, 280 in Austria. This is the hour the control system sells, or the one it avoids buying.
Spot price in euros per megawatt-hour. Same scale on both sides, up to 280.
Hourly spot prices for Sunday 20 September 2026, recorded at 17:00 and 19:00. Source: RTE éCO2mix, EPEX SPOT SE and OMIE. One real day, quoted as it stands: no average, no projection, no promise of gain.
Before signing anything
01
Power readings at ten-minute or quarter-hour intervals, over a full year. Without it, no sizing is serious.
02
The exact structure of the price: capacity share, energy share, time bands, overrun penalties.
03
The mechanisms open to on-site storage, and the conditions of access. They differ from one country to another, sometimes from one region to another.
04
The connection capacity available, and what the grid operator requires in order to add storage to it.
05
The mechanisms it actually operates in your country, and the share of capacity it ties up.
06
How long the site will remain in operation, and the changes in consumption already decided.
Send your load curve and the structure of your contract: the sizing and the layers actually available to you are priced on that basis.