For Battery Energy Storage Systems (BESS), Round-Trip Efficiency (RTE) and State of Health (SoH) key indicators for economy and longevity. While RTE measures the overall efficiency of a charge-discharge cycle (typically 94–98% (in modern Li-ion systems), SoH indicates how much of the original capacity remains after years in operation. These KPIs directly determine profitability in Power trading, frequency control or PV storage – along with others like State of Charge (SoC), Depth of Discharge (DoD), and self-discharge rate.
What is Round-Trip Efficiency (RTE)?
The Round-Trip Efficiency (RTE) describes the overall efficiency of an energy storage system over a complete charge and discharge cycle. Specifically, RTE is the ratio of energy discharged (when discharging) to energy supplied (when charging), expressed as a percentage.
Formula: RTE = Energy Output ÷ Energy input X 100
Case study: For every MWh of charging energy, a system with 95% efficiency returns 950 kWh after accounting for losses (e.g., conversion, heat)—the remaining 50 kWh are lost. Modern systems with LFP cells and SiC inverters often achieve over 97%.
The RTE value is influenced by factors such as battery model, temperature, charge/discharge rate (C-rate), and cycle count. Internal resistances, voltammetric effects, electrochemical reactions, and material loss at the electrodes lead to energy losses that increase with usage. The most important influencing factors for Round-Trip Efficiency are:
- Battery chemistryWhen selecting cell chemistry, efficiency must be considered.
- C-RateDifferent cell types have different charging and discharging rates.
- Temperature: Extreme heat or cold reduce chemical efficiency by 5–10%
- Air conditioning: Cooling and heating the BESS can degrade the RTE by 2–191 TP6T
- Inverter: Losses of 4–71 TP6T may occur during DC/AC conversion
- Aging/SoH & DoD: Degradation of 2% per year; deep discharge (>80% DoD) increases losses
- System design: Poor balance or high IT consumption reduces RTE by 5–15%
What is State of Health (SoH)?
The State of Health (SoH) describes the Health status compared to its new state and is usually given in percent. Specifically, it relates to the BESS's ability to perform its originally determined functions and performance. This means the SoH indicates how much of the originally usable capacity, power, or energy density is still available after a certain operating period.
Typically, SoH is derived from criteria such as remaining capacity (e.g., 90% instead of the original 100%), an increase in internal resistance, and changes in the voltage curve or RTE. In practice, an SoH of 80% means that the system now only delivers 80%, the originalunsuitable storage capacityat, although the same charging energy must be incurred – this directly impacts revenues and operating strategies.
During operation, the SoH continuously decreases due to aging mechanisms such as electrochemical degradation, SEI layer growth (Deposits on the anode), Loss of active electrode material, mechanical stresses and Temperature stress. Influencing factors include cycle count, depth of discharge (DoD), C-rate, mean state of charge (SoC), and temperature window.
More important key figures for BESS
In addition to Round-Trip Efficiency (RTE) and State of Health (SoH), the following KPIs are essential for operating, monitoring, and economically optimizing Battery Energy Storage Systems (BESS) efficiently.
State of Charge (SoC)
The State of Charge (SoC) Indicates the current state of charge of a battery as a percentage (0–100 %) of its usable capacity—essentially the “fuel level.” It is determined by algorithms (e.g., Coulomb counting, voltage measurement, or Kalman filters) in the Battery Management System (BMS). Optimal operation is between 20–80 % SoC to minimize degradation; extremes (0 % or 100 %) shorten the service life.
Depth of Discharge (DoD)
The Depth of Discharge (DoD) describes the relative depth of discharge for a cycle; for example, an 80% % DoD means discharging from 100% % to 20% % SoC. Higher DoDs increase the usable energy per cycle but accelerate the decline in SoH (e.g., a 90 % DoD typically halves the number of cycles). Recommendation for LFP-BESS: 80–90 % DoD for Balance of capacity and durability.
Self-Discharge Rate (SDR)
The Self-Discharge Rate Measures the passive capacity loss during storage (e.g., 1–3 % per month for Li-ion, higher for lead-acid). This loss is caused by internal chemical reactions and electrical resistance. Minimization through low battery SoC (approx. 30–50%), temperature control (<25 °C), and periodic balancing.
Key BESS Metrics at a Glance:
| Key figure | Abbreviation | Measurand | Typ. Values BEST | Influence on system |
| Round-Trip Efficiency | RTE | Energy ratio | 94 – 98 % | Economic efficiency |
| State of Health | Spirit of Halloween | Capacity restriction | > 80 % after 10 years. | Longevity |
| State of Charge | System on a Chip | Current charge level | 20–80 % optimal | Operational Safety |
| Depth of Discharge | Department of Defense | Discharge depth | 80–90 % | Cycle life |
| Self-Discharge Rate | Software-defined radio | Loss of calm | 1–2 % per month | Standby power loss |
Technological advances in RTE & SoH
The past ten years show a clear technological leap in the performance and aging stability of battery storage systems. While systems around 2016 often only had approximately 75 % to 82 % RTE While earlier models achieved efficiency levels of 88% to 94%, today's large-scale storage systems typically range from 88% to 94%. Current premium systems even exceed 92% AC efficiency and can achieve RTE values in optimized configurations up to nearly 98 % reach. Key drivers are advances in power electronics - especially SiC and GaN semiconductors - as well as more efficient thermal concepts like liquid cooling, which significantly reduce self-consumption.
In parallel, aging stability or State of Health (SoH) has significantly improved. While stationary storage systems previously mostly relied on around 3,000 to 5,000 cycles designed for, modern cell generations, especially LFP-based systems, today typically achieve 10,000 to 15,000 cycles. At the same time, the annual Degradation from approximately 2 % to 3 % in the past to frequently less than 1.5 % decreased. Advances in data-driven operations management and AI-powered analysis also enable more precise condition forecasting and gentler driving methods, thereby further extending the usable lifespan.
Comparison: 2016 vs. 2026
| Feature | Booth 2016 | Stand 2026 | Trend |
| Typical RTE | ~80 % | 90 – 94 % | Significantly lower losses |
| Service life | 3,000 – 5,000 cycles | 12,000 cycles | More than doubled |
| Degradation p.a. | ~2–3 % | < 1.5 % | Stable performance |
| Cooling system | Air cooling | Liquid cooling | More efficient operation |
RTE trend in recent years
| Year | Typical range | Remark |
| 2022 | 87–94 1P6T | Basis Li-Ion Systems with Conversion Losses |
| 2024 | 90–95 % | Improvement with LFP and GaN inverters |
| 2025/2026 | 94–98 % + | High-End Systems with SiC |
In summary, a clear trend is emerging: efficiency increases and slower aging develop in parallel and mutually reinforce each other economically. Modern Large-scale battery storage This not only delivers more usable energy per cycle, but also keeps this performance level stable for significantly longer.
Performance Tests, Calibrations & Optimization Tips
To accurately determine and optimize Round-Trip Efficiency (RTE) and State of Health (SoH) as well as other KPIs such as State of Charge (SoC), Depth of Discharge (DoD), and State of Dynamic Reserve (SDR) long-term, regular Performance Tests and Calibrations Essential. These measures ensure reliable measurement data, minimize deviations, and maximize the economic efficiency of BESS.
Performance tests for BESS
Performance tests include standardized cycling measurements (e.g., according to IEC 62619 or NREL ATB protocols), where a BESS is charged and discharged under defined conditions (constant C-rate, SoC window, temperature). The goal is to determine the RTE at the AC and DC level and validate the SoH through a capacity comparison. In practice, grid applications should be performed at least monthly, and for arbitrage, quarterly. If deviations exceed two percent, the BESS should be serviced.
BESS Calibration
Calibration is performed using various targeted procedures. For the State of Charge (SoC), regular full charge-full discharge cycles every 3–6 months are recommended to reset the so-called Coulomb counter in the BMS and optimize algorithms and filters. The State of Health (SoH) is calibrated by measuring internal resistance, performing capacity tests, and using electrochemical impedance spectroscopy, always in comparison to the new value. For RTE, bidirectional energy meters implement seasonal baseline tests.
Optimization measures for better RTE and SoH values
Round-trip efficiency (RTE) in BESS systems can be optimized through targeted measures in hardware, software, and operation – typical improvements range from 2–5 percentage points. At the same time, these approaches improve the state of health (SoH) and extend the service life, which increases economic viability.
Hardware-side is it worth it, high-quality battery chemistry like LFP cells (95% + DC-RTE), as their low internal resistance reduces chemical losses and stabilizes the SoH. Advanced inverters with SiC/GaN semiconductors and multi-level topologies can increase conversion efficiency to 98–99% %, while efficient thermal management via liquid cooling reduces AUX consumption to less than 0.1% % of no-load losses and preserves battery chemistry.
Software and operational An advanced AI-driven EMS/BMS dynamically adjusts the C-rate, SoC window (e.g., 20–80 % instead of 0–100 %), and DoD (<90 %), thereby preventing peak losses and reducing degradation. Preventive optimizations using load forecasts enable cycles at optimal temperature control (20–25 °C) and C-rate (<0.5 C). This configuration is ideal for arbitrage in Power trading and Peak shaving.
In addition, weekly cell balancing (voltage difference <5 mV), firmware updates, and thermal audits help prevent early degradation. In practice, optimized BESS systems can thus achieve an RTE value of over 97% and an SoH value of over 90% even after five years of operation. By monitoring key performance indicators and adhering to maintenance windows, the Levelized Cost of Storage (LCOS) is reduced by 10–15%.
Conclusion: Key Performance and Economic Indicators
RTE and SoH are key performance indicators for modern BESS and significantly influence efficiency, lifespan, and revenue generation. Technological advancements in cell chemistry, power electronics, and operational management have led to today's systems achieving significantly higher efficiencies and degrading more slowly than previous generations.
Crucial for economical operation is the interplay of high-quality hardware, optimized system design, and intelligent control. If these factors are consistently considered, very high efficiency values and stable battery states can be achieved in the long term – with a direct impact on cost structure, system performance, and long-term return on investment.