So far, the summer has been generous. Clear skies and temperatures above 30°C have already arrived in May. While such weather conditions result in high photovoltaic (PV) generation and support the decarbonization of the power system, they also create new challenges for grid operators.
One of the most significant effects is the increasing steepness of PV ramps. During the morning and afternoon hours, PV generation in Germany can change by several gigawatts within just 15 minutes.
Since the introduction of quarter-hourly day-ahead auctions, these generation changes can be efficiently traded by aggregators and direct marketers in the spot market. As a result, balancing groups can remain largely balanced on a quarter-hour basis.
However, an important question remains:
What happens within the quarter-hour?
The Challenge of Intra-Quarter-Hour PV Ramps (simplified)
The figure below shows PV generation in Germany on May 26, 2026, including zoomed-in views of the morning and afternoon ramp periods and a simplified assumption of traded volumes.
Energy is traded and settled in 15-minute intervals. Direct marketers forecast the energy production of PV assets for each quarter-hour and sell the corresponding energy volume. The traded energy volume represents the average power output during the respective interval.
As PV penetration continues to increase, generation ramps become steeper. Naturally, these ramps do not align with quarter-hour boundaries. As a result, systematic over- and under-delivery occurs within each quarter-hour, even when the total energy delivered over the interval matches the forecast.
The morning ramp illustrates this effect particularly well.
Between 09:00 and 09:15, the forecasted PV generation amounts to 7,5 GWh, corresponding to an average traded power output of 30 GW. In reality, PV generation starts at approximately 28 GW at 09:00 and rises continuously to around 32 GW by 09:15.
This means:
- During the first half of the quarter-hour, more power was sold than actually generated.
- During the second half of the quarter-hour, less power was sold than actually generated.
From an energy perspective, the quarter-hour is balanced. From a power perspective, significant intra-quarter-hour imbalances occur.
During the afternoon ramp-down period, the same mechanism appears in reverse.
The Role of aFRR
These intra-quarter-hour imbalances cannot currently be traded in the spot market because settlement is based on quarter-hourly energy values. Direct marketers therefore fulfill their market obligations as accurately as possible, while the remaining imbalance must be managed by the system operator.
To handle such short-term deviations, transmission system operators (TSO) procure balancing reserves. Automatic Frequency Restoration Reserve (aFRR) is particularly well suited to compensate for these ramping effects.
During an upward PV ramp, a power deficit occurs in the first half of the quarter-hour, requiring positive aFRR energy. In the second half, a power surplus emerges, resulting in the activation of negative aFRR energy.
For downward PV ramps in the afternoon, the pattern reverses.
Battery Storage as the Balancing Mechanism
Entelios currently operates a battery portfolio exceeding 1.5 GW of installed capacity. Most of these assets also participate in the aFRR market.
Due to their low marginal costs and fast response times, battery energy storage systems (BESS) are typically positioned at the front of the merit order and are therefore among the first assets activated by the TSO.
We observe this phenomenon regularly across our portfolios. On sunny days, battery systems frequently receive negative aFRR energy activations during the morning ramp-up period and positive aFRR energy activations during the afternoon ramp-down period, reflecting the intra-quarter-hour imbalance pattern described above.
The examples below show typical activation profiles from one of our BESS pools operating in a German control area.
Conclusion: Battery storage closes the operational gap
Battery energy storage systems play a crucial role in maintaining secure system operation. They enable TSOs to efficiently compensate for the intra-quarter-hour imbalances created by rapidly changing photovoltaic generation.
As solar capacity continues to expand, these ramp effects will become increasingly pronounced. Batteries provide the flexibility needed to bridge the gap between quarter-hourly market design and the continuous physical requirements of the power system.
