Usable Capacity

Gross vs usable

Battery manufacturers advertise two different capacity figures, and conflating them leads to confusion when comparing products.

Gross (nominal) capacity is the total energy the battery cells could theoretically store if charged to absolute maximum and discharged to zero. This figure appears in the product name in some cases - though modern products tend to market the usable figure directly.

Usable capacity is what the BMS actually makes available after applying its protection margins. Charging to 100% of cell voltage and discharging to 0% accelerates wear significantly, so the BMS limits both ends. For most home batteries, the usable figure is 90–100% of gross, depending on chemistry and product design.

LFP chemistry handles deep cycling well, so many LFP products publish usable capacity equal to or very close to gross capacity. Some manufacturers set usable = gross and simply absorb a slightly faster degradation curve; others maintain a hidden reserve.

Backup reserve further reduces daily usable capacity

If you configure a battery to hold a reserve for grid outages - say 20% - your daily usable capacity drops accordingly. A 10 kWh battery with a 2 kWh blackout reserve delivers 8 kWh for daily solar self-consumption. This is a user setting in most systems, not a fixed hardware constraint.

Getting this wrong is a common miscalculation in DIY battery payback estimates. People use the advertised usable capacity figure without accounting for the backup reserve they’ve configured, then wonder why the battery runs out before their expected time.

Degradation reduces usable capacity over time

All batteries lose capacity with age. A 10 kWh battery warranted to 70% of original capacity at end of warranty will deliver a minimum of 7 kWh usable after that period - assuming it’s degraded to the warranted floor. In practice, most batteries degrade more slowly than the warranty floor, but any financial model for battery payback that assumes constant capacity over 10 years is overstating the benefit by a few percent.

Stacking batteries

Most current residential systems are modular - you add battery units to increase total usable capacity. Sungrow’s SBR series stacks from 9.6 kWh to 25.6 kWh. BYD’s Battery-Box HVM stacks to 22.1 kWh. If your initial installation is undersized, expansion is usually possible without replacing the inverter, provided the inverter’s battery input spec supports the additional capacity.