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Battery Bank Size Calculator

Battery bank size calculator: size amp-hour capacity from load watts, runtime hours, and system voltage. Plan off-grid, RV, and backup banks with clear Wh → Ah steps—free, instant.

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Ah sizing is an energy-budget problem: how many watt-hours you need, divided by bus voltage. This guide walks through the battery bank size calculator logic before you buy cells, cables, or fuses.

Benefits

  • Transparent chain: Wh needed = load (W) × hours; Ah = Wh ÷ voltage—reproducible on a napkin or in the tool.
  • Supports 12 V, 24 V, and 48 V buses common in RV, marine, and small off-grid installs.
  • Detail output shows intermediate Wh so you can cross-check against solar-battery-bank planners.

How it works

  1. Enter total load power in watts and the hours of autonomy you need.
  2. Set nominal system voltage (12, 24, or 48 V typical).
  3. Review required Ah; multiply by 1.2–1.5 for inverter loss, depth-of-discharge limits, and aging.

FAQ

How do I calculate battery bank size in Ah?

Multiply load watts by runtime hours to get watt-hours, then divide by system voltage. Example: 500 W × 8 h = 4,000 Wh; at 12 V that is about 333 Ah before safety margin.

Should I size at 12 V or 48 V?

Higher voltage reduces current for the same watts, which can shrink cable size. Ah at the battery is still Wh ÷ V—compare total Wh needs first, then pick a bus architecture your inverter and charger support.

Do I need extra capacity beyond the calculator result?

Yes. Add margin for inverter efficiency, temperature, and not discharging to 100% depth of discharge. Many installers use 1.2–1.5× on the raw Ah figure unless a specific DoD target is modeled.

Technical specifications

  • Formula: Ah = (load_W × hours) ÷ voltage_V.
  • Inputs: positive W, hours, and V; assumes near-constant load.
  • Planning margin: ×1.2–1.5 recommended for loss and DoD unless detailed model applied.
  • Related: pair with solar-battery-bank and inverter-sizing tools for full system design.

Wh first, Ah second

Capacity in amp-hours only makes sense at a stated voltage. Two banks with the same Ah at 12 V and 24 V store different energy. Always compute watt-hours from load and runtime, then convert to Ah for the bus you are wiring.

Parallel strings and nameplate Ah

Field banks combine series strings for voltage and parallel strings for capacity. The calculator gives total bank Ah at system voltage—match that to how vendors sell rack or floor-mounted modules before ordering interconnects.