Solar Battery Bank Calculator
Size a battery bank properly: not just for the energy it must store, but for the current it must deliver. Enter the daily backed-up energy, the days of autonomy and the peak load, choose the chemistry and battery unit, and the calculator returns the required battery energy, the Ah capacity, the real autonomy achieved, the maximum discharge current, and the series-parallel arrangement — stating whether energy or C-rate governed the sizing.
Establish the critical load with the Load Schedule Calculator, size the array with the Panel Quantity and Roof-Area Calculator, and take the investment view with the Solar Payback and ROI Calculator.
Battery bank schedule CALC-14 · REV 2026-07
| Required stored energy | – |
|---|---|
| Series-parallel arrangement | – |
| Bank energy (installed) | – |
| Bank capacity | – |
| Actual autonomy achieved | – |
| Maximum discharge current | – |
| Discharge rate on the bank | – |
| Governing sizing criterion | – |
| Recommended charge current | – |
| DC protection guidance | – |
Guidance tool only. A real bank design must follow the manufacturer’s series/parallel limits and BMS requirements, temperature derating, cable and busbar sizing for the discharge current, DC arc-rated protection, and ventilation (hydrogen, for flooded cells) — work for a competent solar designer.
How this calculator works
- Stored energy. The bank must hold the daily energy times the autonomy, divided by everything that eats into it: depth of discharge (85% for LiFePO4, 50% for lead-acid), the battery round-trip efficiency (95% / 85% / 80%) and the inverter efficiency. That divisor is why a ‘10 kWh’ requirement becomes a 13 kWh lithium bank — or a 25 kWh lead-acid one.
- Series, then parallel. The series count builds the system voltage from the unit voltage (a 51.2 V rack unit serves a 48 V bus directly; four 12 V units in series do the same). Parallel strings then multiply capacity — sized by the larger of two criteria: the energy requirement, and the discharge current the peak load draws at the DC bus against the chemistry’s C-rate limit (1C lithium, 0.2–0.25C lead-acid). The calculator states which one governed.
- The real numbers. Because units are discrete, the installed bank always exceeds the requirement — the actual autonomy, usable energy, bank Ah and resulting C-rate are reported, along with the recommended maximum charge current (about 0.5C lithium, 0.1–0.2C lead-acid) and DC protection sized at about 1.25 times the peak discharge.
Assumptions and limits
- Nominal capacity at 25 °C; lead-acid loses capacity at high discharge rates (Peukert effect) and both chemistries derate outside their temperature window.
- Manufacturer series/parallel and BMS limits are binding — many lead-acid makers cap parallel strings at four.
- Charger/MPPT sizing against the array, cable and busbar design, and arc-rated DC protection are part of the full design.
Want the complete hybrid design — array, bank, protection and changeover? See our consultancy services, or learn the method in a training program.