As the global energy transition accelerates, large-scale battery energy storage systems (BESS) have become essential for grid stability, renewable integration, and energy trading. At the heart of these systems next to the battery cells lies a critical but often underappreciated component: the inverter.
Inverter efficiency significantly impacts the overall performance, RTE (Round-Trip Efficiency) and profitability BESSs. This article explores the types of inverter losses, their effect on efficiency, and how smart analytics can help improve system performance.
The Role of Inverters in BESS
BESSs have become essential for balancing supply and demand, stabilizing the grid, and enabling energy trading. The inverter plays a foundational role, enabling grid connection and compliance with market requirements.
Since batteries store energy as direct current (DC) and most grids operate on alternating current (AC), inverters are responsible for two-way conversion. In other words, BESS inverters are bidirectional, meaning they can both:
- Invert DC to AC when discharging the battery power to the grid
- Rectify AC to DC when charging the battery from the grid or a renewable source
This bidirectional capability allows the battery system to respond dynamically to grid needs, market signals, or local consumption patterns. However, inverters can do much more than current conversion; they act as the gate to markets and grid connection permits. They’re also responsible for:
Grid Compliance and Power Quality:
Inverters ensure that the power delivered to the grid meets strict standards for voltage and frequency, active and reactive power. They can operate in grid-following mode, where they synchronize with the grid and give support, or grid-forming mode, where they help stabilize the grid even during disturbances or blackouts.
Real-Time Control and Monitoring:
Inverters are equipped with sensors, communication interfaces, and control algorithms that allow monitoring and control of voltage, current, and temperature, remote diagnostics and firmware updates, and integration with energy management systems (EMS).
Integration with Energy Management Systems:
The inverter works closely with the EMS to execute dispatch schedules, optimize charge and discharge cycles, respond to market prices or grid signals, and to minimize energy losses and battery degradation. This coordination ensures that the battery system operates efficiently and profitably.
Round-Trip Efficiency (RTE):
RTE is, alongside capacity (SoH) and technical availability guarantees (TAG), one of the most critical warranty parameters for BESS. Inverter inefficiencies typically range from 1–4%, making them a significant contributor to overall system losses. As such, inverter performance is a key determinant of the system’s RTE.
Inverter Losses and Efficiency
Like any electrical component, inverters experience electrical losses. However, their efficiency varies widely depending on the output power, forming a curve that resembles a square root—low efficiency at partial loads and higher efficiency at medium to high power. Especially the partial load losses depend on the specific model (see Figure 1).
Key Loss Types
There are different types of losses in inverters which lead to the efficiency curve above. At low power levels, standby and switching losses dominate, leading to poor efficiency. As power increases, these losses become a fraction of the total power, and efficiency improves until ohmic losses begin to rise steeply. All in all, the most important losses are as follows:
- Standby & auxiliary losses in inverters refer to the power consumed when the inverter is powered on but not actively converting energy. These losses come from internal components like control electronics, gate drivers, and cooling systems that remain operational even during idle periods. Although small in absolute terms, standby losses can significantly impact overall system efficiency, especially in applications with frequent low-power operation or long standby durations. Figure 2 depicts both standby and auxiliary losses at the example of a high-power charge where the inverter needs to be cooled.
- Ohmic (Conduction) losses in inverters occur due to the electrical resistance in components like transistors, diodes, and internal wiring. These losses increase with the square of the current flowing through the inverter, thus they increase significantly with higher power output. As current rises, more energy is dissipated as heat, reducing overall efficiency. Ohmic losses are especially relevant during high-load operation and can be minimized through the use of low-resistance materials and optimized circuit design. Figure 2 depicts the quadratic relationship between current and ohmic losses.
- Switching losses in inverters occur during the brief transitions when semiconductor devices like IGBTs or MOSFETs switch between on and off states. During these transitions, both voltage and current are present across the device, causing energy to be dissipated as heat. These losses increase with switching frequency and are also influenced by the load current—meaning they grow approximately linearly with output power. Figure 4 depicts the switching losses. In addition, other losses, such as dead-time losses occur due to a slight delay between turning on one switch after turning off another one.
Efficiency in Different Use Cases
Different BESS markets lead to different operational power levels, which directly affect inverter efficiency. Two common use cases that can be seen opposite in power provision are frequency regulation and wholesale trading as shown in Table 1.
- Frequency regulation services require rapid, bidirectional power adjustments often at low power levels. These low power levels occur as the frequency deviations are typically only a fraction of their critical limits in normal grid operation. As a consequence, inverters operate in partial load, and average efficiency often is below 90% just for the inverter. This inefficiency leads to high losses.
- Wholesale Trading, in contrast, typically involves full charge and discharge cycles at high power. Inverters operate near their peak efficiency (97–98%), maximizing energy throughput and revenue.
Table 1 summarizes and compares the operational power and efficiency for the two markets under the assumption that the BESSs are operated in single-use operation.
These differences underline why operators often serve both markets simultaneously in multi-use operations. Here, reduced wholesale trading power can be overlayed with a frequency regulation profile to serve both markets as needed. Variations in partial-load vs. full-load efficiency directly translate into differences in RTE, with low-load services often dragging down the overall system round-trip performance.
Leveraging Data Analytics to Maximize Efficiency
Modern BESS generate vast amounts of operational data. This data can be used to identify if the inverter operates as it should. By applying data analytics, operators can both enhance their power distribution, and track aging and efficiency before it leads to failure.
Identify failure before it happens: Proactively identifying inverter issues before shutdown events is essential to maintaining availability and protecting the financial performance of a BESS.
When inverters shut down unexpectedly, the total power available for dispatch decreases. This reduction directly impacts the energy that can be traded on the market, resulting in lost revenue and potential penalties for underdelivery. Additionally, it lowers the system’s technical availability, thereby affecting the Technical Availability Guarantee (TAG). Predicting inverter failures in advance allows operators to plan maintenance activities before problems escalate. By continuously monitoring indicators such as load imbalances, asymmetric operation, and subtle performance deviations, it is possible to detect issues early and keep the system running at full capacity. Understanding the root causes of these failures supports long-term reliability improvements by preventing repeated issues and enhancing operational resilience across the entire fleet. By preventing unexpected efficiency drops, operators can stabilize RTE over the system lifetime, which is essential for both warranty compliance and profitability.
Optimize power distribution: Dynamically allocating power across multiple inverters is essential for maintaining balanced operation and maximizing overall system efficiency. Identifying inverter behavior anomalies, such as asymmetric operation across DC channels, allows operators to detect early indicators of control logic faults, thermal imbalances, or uneven cycling. Addressing these issues proactively ensures that power is distributed evenly, reduces unnecessary wear on individual components, and maintains high system efficiency throughout the entire lifecycle of the asset.
In addition, when distributing power signals, the inverter-specific efficiency curve should be considered. For example, if a 100 MW BESS with twenty 5 MW inverters is tasked with delivering 10 MW, it’s more efficient to use 2–3 inverters at 67–100% load rather than running all inverters at 0.5 MW. This minimizes partial load losses. Such a distribution is schematically explained in Figure 3, where only one inverter provides higher power in Option A at high efficiency and the same amount of power is provided in Option B with low power in partial load.
Track aging and efficiency decrease: Monitoring the aging and efficiency of inverters is essential for maintaining reliable performance and maximizing the return on investment in energy systems. This process involves continuously tracking key operational parameters such as input/output voltage and current, switching frequency, temperature, and harmonic distortion. Over time, deviations from baseline performance, like increased power losses, reduced conversion efficiency, or abnormal thermal behavior, can indicate component degradation. Advanced monitoring systems use data analytics and machine learning to detect early signs of aging, enabling predictive maintenance before failures occur. Additionally, efficiency trends can be analyzed to assess the inverter’s health and optimize its operation, ensuring that energy losses are minimized and system uptime is maximized.
Conclusion: Efficiency is Profitability
Inverter efficiency is more than a technical detail. It’s a key driver of profitability and sustainability in large-scale battery storage systems as it reduces the power that can be traded. From choosing the right inverter architecture to optimizing dispatch strategies with data analytics, every percentage point of efficiency matters.
At ACCURE, we specialize in helping energy storage operators unlock the full potential of their systems. Our advanced analytics platform and engineering expertise ensure that your inverters and batteries operate at peak efficiency, maximizing returns and minimizing losses.
