As the energy storage industry continues to expand in Poland, more and more calculations and analyses are popping up online. You might have come across various abbreviations, assumptions, and technical lingo. To help everyone speak the same language and ensure we all understand each other, we’ve put together this glossary of essential battery and energy storage terms.

Whether you’re a newcomer or a seasoned professional, we hope this helps clarify the jargon and enables deeper discussions about battery energy storage systems (BESS).

Glossary

Capacity (of a Battery)

• Definition: The total amount of energy a battery can store or deliver, measured in kWh or MWh.
• Manufacturer Terminology: When manufacturers label a battery as 4 MW / 16 MWh, they often mean it has 4 MW power and 4-hour duration, leading to 16 MWh usable capacity. However, in reality, the actual capacity of such a battery is 16 MWh divided by discharge efficiency. This means that the real usable energy depends on the efficiency of the discharge process. It is always advisable to confirm the exact values in the manufacturer's datasheet.

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Cycle

• Definition: One complete charge from the allowable minimum to maximum state, followed by a discharge back down.
• Manufacturer Limits: Many battery warranties specify a maximum number of allowed cycles (e.g., 8000 total, or 600 per year).
• Daily Constraints: In optimization models, it’s common to set a maximum number of daily cycles (for example, not exceeding 1 cycle/day if the annual limit is 365 cycles). This limit doesn’t force you to use the battery every day—it just ensures you don’t exceed warranty terms over the year.

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Degradation Curve

• Definition: The gradual loss of capacity and performance caused by chemical and structural changes in battery materials over time.
• Stepwise Degradation: In practice, optimization models often update the battery’s capacity at certain intervals (e.g., monthly or yearly) to reflect the lost capacity, rather than decreasing after each individual cycle.

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Depth of Discharge (DoD)

• Definition: The percentage of total battery capacity that can be discharged in a single cycle.
• Typical Limit: Often set at 90% to preserve battery health. If a battery is 5 MWh, an 80% DoD allows using up to 4 MWh per cycle before recharging.
• Upper Charging Limit: Some warranties also place a top-end limit on charging (e.g., 95–97% of battery capacity) to avoid stress on the battery’s chemistry.

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Duration (of a Battery)

• Definition: Describes how many hours a battery can operate with nominal power, starting from full charge and ending at complete discharge. The same applies for the charging activity.

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Perfect Foresight vs. Imperfect Foresight (Price Paths)

• Perfect Foresight: Assumes the battery operator knows all future energy prices with 100% accuracy. This is typically used to find the theoretical maximum revenue the battery could generate.
• Imperfect Foresight: Reflects real-world conditions where future prices are uncertain. The battery optimizes based on expected (but not guaranteed) price signals, potentially resulting in suboptimal decisions compared to perfect knowledge.

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Round Trip Efficiency (RTE)

• Definition: The ratio of energy delivered during discharge compared to the energy consumed during charging, typically expressed as a percentage.
• Example: If a battery is charged with 10 MWh and delivers 8.5 MWh, it has an RTE of 85%.
• Charging & Discharging Efficiency: Some manufacturers provide separate efficiency values for charging and discharging. For example, if charging efficiency is 93% and discharging efficiency is 93%, the total RTE = 93% × 93% = 86.49%.

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State of Charge (SoC)

• Definition: The current capacity a battery has available, expressed as a percentage of its maximum.
• Practical Note: If the SoC is 25%, it means the battery has a quarter of its usable capacity remaining.

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State of Health (SoH)

• Definition: An indicator of a battery’s overall condition relative to its original (rated) specification.
• Key Insight: A battery with an SoH of 85% can hold less charge and deliver slightly reduced performance than when it was brand new.
• Manufacturer Guarantee: Some manufacturers specify that after 8000 cycles, SoH should not be lower than 70%, ensuring a predictable degradation rate.

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Why Terminology Matters

With so many analyses, business cases, and technical discussions out there, using consistent terms ensures clarity and mutual understanding. No matter if you’re building a revenue model, comparing battery bids, or simply catching up on the latest industry trends, a shared vocabulary goes a long way toward making solid, informed decisions.

If you have any questions or want to dig deeper into any of these concepts, feel free to reach out to us at contact@flexplum.com!

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