Formulas, depth-of-discharge figures and worked examples for sizing storage correctly.
Core formula
Nominal capacity (kWh) = energy needed during backup ÷ usable depth of discharge. Energy needed = average load × backup hours.
Usable depth by chemistry
- Lithium (LiFePO₄): about 90%
- Gel: about 50%
- Tubular: about 50%
Worked example
Need 6kWh over your backup window on lithium: 6 ÷ 0.9 ≈ 6.7kWh nominal, so a 7kWh+ battery. On tubular: 6 ÷ 0.5 = 12kWh nominal for the same usable energy.
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