Battery Bank Sizing Calculator (Solar/Backup)
Sizes a battery bank in Ah/kWh from daily load, autonomy days, depth of discharge, and system voltage
Sizes an off-grid or backup battery bank using the standard stand-alone power system method: usable energy (daily load × autonomy days) is scaled up for the chemistry's recommended depth of discharge and system conversion losses to arrive at required raw capacity, then translated into a battery Ah rating and a series/parallel string configuration.
How battery bank sizing works
Off-grid and backup battery systems are sized around the energy the load needs during the specified number of autonomy days without recharge, then inflated to raw (nameplate) capacity by accounting for how deeply the chemistry can safely be discharged and how much energy is lost to inverter, wiring, and temperature effects between the battery terminals and the actual load.
Frequently asked questions
Why does LiFePO4 use a higher depth of discharge than lead-acid?
LiFePO4 cells tolerate much deeper cycling with far less cycle-life impact than flooded/AGM lead-acid, which degrades quickly past 50% DoD — manufacturer cycle-life curves are the basis for the 80% vs 50% defaults here.
How many days of autonomy should I use?
Typical off-grid solar designs use 1–3 days for reasonably sunny climates and 3–5+ days for critical loads or areas with frequent multi-day cloud cover; backup/UPS-style systems for grid outages are often sized to a few hours to a day.
What system efficiency should I assume?
85–90% round-trip is typical for a well-wired system with a quality inverter; use a lower value (75–80%) for long wire runs, older/less efficient inverters, or cold-climate installations where battery capacity derates.