Turn nameplate capacity into real usable energy quickly. Model losses, reserve, and depth limits safely. Plan batteries with confidence for every project and budget.
| Scenario | Nominal kWh | DoD | Reserve | Eff. (RT × Inv) | Derate (Deg + Temp) | Load kW | Usable kWh (approx.) | Runtime (h) |
|---|---|---|---|---|---|---|---|---|
| Home backup | 13.5 | 80% | 15% | 0.95 × 0.92 | 5% + 3% | 2.0 | 7.7 | 3.9 |
| Small solar storage | 10.0 | 90% | 10% | 0.96 × 0.94 | 3% + 2% | 1.5 | 7.4 | 4.9 |
| EV pack planning | 60.0 | 70% | 5% | 0.95 × 0.90 | 8% + 0% | 15.0 | 31.2 | 2.1 |
Teams often budget from nameplate kilowatt-hours, yet delivered energy is lower after reserve and conversion losses. For example, a 10.0 kWh pack with 80% depth of discharge and 10% reserve yields 7.2 kWh before efficiency. At 95% round‑trip and 92% inverter efficiency, delivered usable energy is about 6.29 kWh, changing cost and sizing decisions.
A five‑point efficiency swing matters. On a 13.5 kWh home battery, holding DoD at 80% and reserve at 15%, moving combined efficiency from 0.90 to 0.95 increases usable energy by roughly 5.6%. Over 300 cycles per year, that extra energy can offset more grid purchases and improve return when time‑of‑use spreads are tight.
Degradation and temperature derate reduce effective capacity over time. If a system is modeled at 8% degradation and 5% temperature derate, the usable pool shrinks by 12.6% before DoD and reserve are applied. Including derating avoids overstating service levels for critical loads and reduces the risk of costly mid‑life augmentation. For commercial storage, using conservative derates can also align warranty capacity thresholds with financing covenants and service contracts over time.
Runtime is a simple ratio: usable kWh divided by average kW. A site drawing 2.0 kW from 7.7 usable kWh can expect about 3.85 hours, while 1.5 kW extends runtime to roughly 5.13 hours. Compare the load to the maximum discharge limit to avoid optimistic runtime when power caps trigger early shutdown.
Purchase price alone can mislead across chemistries and inverter paths. If a $4,000 battery delivers 6.29 usable kWh, the cost is about $636 per usable kWh. Another $4,600 unit delivering 8.0 usable kWh costs $575 per usable kWh and may be the better financial choice even before valuing longer runtime and reduced cycling depth.
Usable capacity is the energy you can realistically deliver after applying depth‑of‑discharge limits, reserve, derating, and efficiency losses. It is usually lower than the nameplate kWh printed on the battery.
Reserve keeps a buffer for unexpected outages, battery longevity, and inverter cutoffs. A 10–20% reserve is common for backup systems, while some EV packs already include a built‑in manufacturer buffer.
Use both when your load is AC. Round‑trip efficiency captures battery charge/discharge losses, while inverter efficiency captures DC‑to‑AC conversion losses. Multiply them to estimate delivered energy.
Degradation reduces capacity as the battery ages. Temperature derate reflects performance drops in cold or high‑heat conditions. Applying both helps avoid overestimating runtime during real operating periods.
High loads can hit discharge power limits, voltage sag, or BMS protection, causing early cutoff. Also, loads vary over time. Use an average load and check that it stays under the max discharge limit.
Convert each option to cost per usable kWh and expected runtime at your load. This normalizes different chemistries, warranties, and inverter paths, making it easier to justify the best value for your project.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.