• Rated Capacity (also called total capacity) refers to the maximum theoretical energy a battery can store when fully charged, typically measured in kilowatt-hours (kWh). It is the "design limit" listed in product datasheets, reflecting ideal conditions.

• Usable Capacity (or net capacity) is the actual energy that can be discharged during operation. It accounts for safety margins, system limitations, and Depth of Discharge (DOD)—a key factor in balancing performance and battery longevity.

For example, a 100 kWh rated battery might offer only 90 kWh of usable capacity if designed with a 90% DOD to extend lifespan.

Key Features of the Two Capacities

• Design Focus: Rated capacity helps compare total system potential, while usable capacity guides real-world application planning (e.g., estimating backup duration).

• Lifespan Impact: Limiting usable capacity (e.g., 80% DOD) reduces cycle stress, extending the life of lithium-ion batteries.

• Efficiency Factors: Inverter losses, internal resistance, and temperature further reduce usable capacity from the rated value.

• Customization: Some manufacturers let users adjust DOD and reserve margins to meet project-specific needs.

Applications Across Sectors

• Residential ESS: Usable capacity determines how long appliances can run during outages.

• Commercial & Industrial: Influences ROI, peak shaving, and energy arbitrage results.

• Renewable Integration: Ensures storage matches solar/wind output patterns for stable operation.

• Grid Services: Precise usable capacity dictates eligibility for frequency regulation or demand response programs.

Pricing Based on Capacity

ESS costs depend on both rated and usable capacity:

• A system with 100 kWh rated capacity and 90 kWh usable capacity is often cheaper than one with 100 kWh usable capacity, as lower DOD reduces cycle stress.

• Pricing is typically quoted based on usable capacity to reflect real-world performance.

• Other factors: battery chemistry (e.g., LFP vs. NMC), system configuration (rack, containerized), and cooling methods (air-cooled vs. liquid-cooled).

Choosing the Right Capacity

To select an ESS:

• Analyze load profiles (daily usage and peak demand).

• Match DOD to usage patterns to balance usable capacity and battery life.

• Account for round-trip efficiency (inverter and thermal losses).

• Choose battery chemistry (e.g., LFP for deeper discharge tolerance).

Need tailored ESS solutions? Dagong ESS specializes in intelligent energy storage systems. Contact us at sales@dagongess.com or via our "Contact Us" page for details.