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Battery runtime calculator for 12V systems

Estimate how long a 12 V battery system may run a load after usable capacity and efficiency are included.

Open Battery Runtime Calculator Browse Electrical

Quick answer

Estimate how long a 12 V battery system may run a load after usable capacity and efficiency are included.

How to do it

  1. Convert Ah to Wh using Ah × V.
  2. Apply usable capacity and efficiency before dividing by load watts.
  3. Real runtime varies with battery condition, temperature and discharge rate.

Worked example

100 Ah at 12 V is 1200 Wh. At 80% usable and 90% efficiency with a 100 W load, runtime is 8.64 hours.

Common mistakes

  • Using full capacity instead of usable capacity.
  • Ignoring inverter efficiency.
  • Forgetting battery age and temperature.

When this is not suitable

Do not treat any quick estimate as a final specification where safety, contracts, regulations or expensive purchases depend on it.

Related next tools

Next useful tools for this task.

ElectricalElectricity Cost CalculatorEstimate running cost from watts, hours and unit price. Useful for quick appliance and load estimates. ElectricalOhm's Law CalculatorCalculate voltage, current, resistance or power from two known electrical values. ElectricalBasic Voltage Drop CalculatorQuick voltage drop estimate from current, length, supply voltage and resistance per metre. ElectricalProspective Fault Current CalculatorCalculate prospective fault current from voltage and Ze, with kA output and job-note friendly wording.

Keep the result together

Use the calculator first, then add the result to a Project Sheet if you need a clean printable summary.

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