The interaction of battery storage systems with wind and solar installations poses challenges for both grid operators and storage system operators. This is because grid congestion can occur, particularly when there is high feed-in from photovoltaic and wind power sites.
If storage systems were to feed electricity into the grid in such situations, they would only add to the strain. That is why the key question is: How can storage systems be operated in a way that they relieve the burden on the grid – while still remaining profitable? ECO STOR, a German-Norwegian planner and developer of large-scale storage projects, has developed its own solution by forecasting times of grid congestion and adjusting the feed-in accordingly.
In this article, you can find out how, from ECO STOR's perspective, grid operators can also contribute to grid congestion management.
ECO STOR designs its large-scale battery storage systems with one key goal in mind: to provide maximum relief to the power grid. But a 103.5 MW storage system that was connected to the grid in May 2025 in Bollingstedt in northern Germany highlights just how complex that goal can be in practice. For a few hours of the year, the storage system found itself competing with local wind and solar installations to feed electricity into the grid. As a result, unrestricted use of the battery storage system was not possible. Together with grid operator Schleswig-Holstein Netz, it was agreed upon to limit the discharge rating during periods of high renewable power generation. While this move indeed stabilized the grid, it also meant that the economic viability of the project was negatively impacted.
To resolve this problem – not just for the storage system in Bollingstedt, but across all of its storage projects – ECO STOR has developed a forecasting tool which combines weather and generation data from wind and solar with historical load values, enabling it to predict grid load realistically. This makes it possible to determine in advance how much electricity can be fed into the grid without causing congestion. At times of high feed-in from renewables, the storage system automatically reduces its discharge rating.
“In order for the storage system to participate in system service markets – particularly the FCR auction (Frequency Containment Reserve), which opens at 8:00am two days before the delivery date – forecasts must be available up to 42 hours in advance,” explains Franz Hartleitner, Energy Data Scientist at ECO STOR. FCR is a primary balancing reserve that automatically compensates for power line frequency deviations within seconds.
The data so far shows that the system works: Necessary redispatch or curtailment measures at the grid connection point have been reduced to a minimum. A redispatch is an intervention by the grid operator in the operational planning of power plants to avoid congestion in the grid. In the case of a curtailment, the feed-in would have to be reduced, as the grid cannot absorb the electricity. Since wind and PV generation can be well forecasted and storage systems are usually operated in an anti-cyclical manner – charging when renewables are being fed in – interventions are only necessary in exceptional cases.
One key insight from this approach is that operating guidelines for grid serviceability will become essential in the future. Given the growing number of large-scale storage projects, this also creates the possibility for grid operators to develop new control instruments that take regional characteristics into account. However, this requires a regulatory framework. Dynamic grid charges could be part of that framework – fees that flexibly respond to the current grid situation. If a storage system charges during times of high electricity generation and thereby helps relieve the grid, it would not have to pay any charges – or could even receive remuneration. If the impact on the grid is neutral, only minimal fees would apply, while grid-straining operations would incur higher charges. Such a model could provide additional certainty for investors, while simultaneously promoting grid stability.
Battery storage systems are a cornerstone of the energy transition and can be integrated into the power grid in a meaningful way. “A study published in June 2025 by Neon Neue Energieökonomik confirms that rigid specifications – such as blanket feed-in restrictions, undifferentiated grid charges and fixed capacity limits – hold back the potential of storage systems and jeopardize their economic viability. Instead, we need finely tuned, dynamic mechanisms that take grid congestion into account while also incentivizing grid-serving behavior. Large-scale batteries are already helping to reduce redispatch costs and with a smart regulatory framework, they could deliver even greater value – without compromising profitability,” says Franz Hartleitner.
“Storage systems aren’t the problem – they’re part of the solution. With clear rules, forward-looking planning and shared responsibility between grid operators, storage system operators and regulators, batteries can be operated in a way that is economically attractive and beneficial to the system at the same time.”