Size the battery bank
The battery bank must carry your loads through nights and cloudy spells (days of autonomy) without going below its safe depth of discharge. Use the Battery bank calculator on this page.
How
- Choose days of autonomy. AS/NZS 4509.2 typical values are about 2 days with an automatic generator, 2 to 3 days with a manual generator, and 4 to 5 days with no generator. Critical loads with no backup may need more.
- Needed energy = daily kWh times autonomy days, divided by inverter efficiency for AC loads.
- Divide by the design DoD (step power-11) and by a temperature factor if the battery room gets cold. One lead acid maker's table shows about 88% capacity at 10 °C and about 73% near -1 °C.
- Convert to amp hours: kWh times 1,000 divided by system voltage.
- Check charge rate limits: lead acid charge current is often limited to about 0.1 of the 10 hour capacity (0.1C10), so a very large array on a small lead acid bank may be wasted.
- Avoid many lead acid strings in parallel. Pick a cell or module size that gives one or two strings at your voltage, or a modular LFP system designed to be paralleled.
Sources
- Stand-alone system sizing (AE 868 Commercial Solar Electric Systems). Penn State University, John A. Dutton e-Education Institute, accessed 2026-11
- Oversized PV array and battery days of autonomy in stand-alone power systems. GSES (Global Sustainable Energy Solutions), accessed 2026-11
- Temperature vs. Capacity: Flooded Lead-Acid Batteries. Rolls Battery, accessed 2026-11
- Battery temperature compensation (blog). Victron Energy, accessed 2026-11