Battery Bank Sizing for Off-Grid and Hybrid Solar

Battery sizing is the most expensive number in an off-grid or hybrid design. Oversize it and the client pays for capacity that never cycles; undersize it and they sit in the dark on a cloudy week.

The core formula

• E_daily — energy served from the battery per day (evening + overnight load for hybrid; full daily load for off-grid).
• N_autonomy — days without solar recharge: 1 for hybrid backup, 2–3 for off-grid in the tropics, more where wet seasons are long.
• DoD — 0.8–0.9 for LiFePO4, 0.5 for lead-acid.
• η_system — inverter + battery round-trip, ~0.85–0.90.

Worked example

Off-grid home, 8 kWh/day, 2 days autonomy, LiFePO4 at DoD 0.85, efficiency 0.88:

Round up to standard modules — e.g. 4 × 5.12 kWh = 20.5 kWh, or 5 modules for margin.

Corrections

• Temperature: capacity falls in cold; lead-acid loses ~20% near 0 °C. Lithium needs a low-temperature charging cut-off.
• Ageing: size for end-of-life capacity (often 80% of initial) if the design must hold up for the full warranty term.
• C-rate: confirm the bank can deliver the peak inverter power — kW draw / bank kWh must stay within the battery’s continuous discharge rating.

Charging

The array and charge controller must replace a full day’s battery draw plus the day’s direct load within the available sun hours — check the array is not undersized for recharge.