Battery Size Calculator

Battery Size Calculator

Estimate required Ah capacity for your backup or solar system.

Enter Your Values

Formula

Energy (Wh) = Load (W) × Hours (h)

Required Ah = Energy ÷ Voltage ÷ (DoD × Efficiency)

Need to size a battery backup? The Battery Size Calculator tells you how big a battery bank (Ah) you need to run specific loads for a target number of hours. Enter device wattages or total power draw, desired backup hours, system voltage (12V/24V/48V), and an efficiency / depth-of-discharge (DoD) factor — the tool returns required amp-hours (Ah) and recommended battery bank configuration. This is essential for off-grid solar, backup systems, UPS planning, and EV accessory sizing. Use it to pick the right battery capacity, avoid under-sizing, and estimate run times accurately.

Definition / Why it matters

A Battery Size Calculator estimates battery capacity (Ah) required to supply a known load for a set time considering system voltage and battery usable capacity. It prevents undersized banks, reduces repeated cycling, and improves long-term battery life.

Formula(s)

  • Energy needed (Wh) = Total Load (W) × Backup Hours (h)

  • Required Ah = Energy needed (Wh) ÷ System Voltage (V) ÷ Usable Fraction (DoD × Efficiency)

    • Example: Ah = (W × h) / (V × DoD × Eff)

  • If using inverter: include inverter efficiency → Effective load = Load ÷ Inverter Efficiency

Inputs

  • Load entry: list of devices (W) OR total power (W)

  • Backup time (hours)

  • System voltage (12 / 24 / 48 V)

  • Depth of discharge (DoD %) (default 50% for lead-acid, 80% for Li-ion)

  • Inverter efficiency (%) (if AC loads)

  • Safety margin % (optional)

How to use (step-by-step)

  1. Add device wattages or enter total load in watts.

  2. Select backup time (hours).

  3. Choose system voltage (12/24/48V).

  4. Enter DoD and inverter efficiency (defaults provided).

  5. Click Calculate → receive required Ah, suggested battery count (e.g., 2 × 12V 200Ah in series/parallel), and estimated runtime.

Quick reference examples (5)

  • 200 W load, 5 h, 12 V, DoD 50% → Wh=1000 → Ah=(1000)/(12×0.5)=166.7 Ah

  • 500 W load, 4 h, 24 V, DoD 80% → Ah=(2000)/(24×0.8)=104.17 Ah

  • 1000 W load, 2 h, 48 V, DoD 80% → Ah=(2000)/(48×0.8)=52.08 Ah

  • 50 W LED, 24 h, 12 V, DoD 50% → Ah=(1200)/(12×0.5)=200 Ah

  • 1500 W inverter, 1 h, 48 V, inverter eff 90%, DoD 80% → Effective Wh=1500/0.9=1666.7 Wh → Ah=1666.7/(48×0.8)=43.45 Ah

Quick table example (short)

Load (W)HoursVoltageDoDRequired Ah
200512V50%167 Ah
500424V80%104 Ah
1000248V80%52 Ah

FAQs

  1. Q: How do I calculate battery size?
    A: Multiply load (W) by hours → divide by voltage and usable battery fraction (DoD × eff).

  2. Q: What DoD should I use?
    A: Lead-acid: 40–50%; Li-ion: 70–90%. Use conservative DoD for longer life.

  3. Q: Does inverter efficiency matter?
    A: Yes — divide AC loads by inverter efficiency to get DC energy required.

  4. Q: How many batteries do I need?
    A: Calculator can output suggested number based on battery Ah ratings and required system voltage.

  5. Q: Should I add a safety margin?
    A: Yes — add 10–20% for contingencies and aging.

  6. Q: Is Ah the same as Wh?
    A: No. Wh = Ah × Voltage. Wh measures energy, Ah measures battery capacity at a voltage.

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