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Battery Runtime Calculator

Find out how many hours your battery can run a given load before it needs recharging.

How it is calculated

The tool first finds the energy you can actually use: usable Wh = capacity (Ah) × voltage (V) × DoD / 100. Depth of discharge (DoD) is entered as a percentage, so 50 means half of the battery. Runtime is then hours = usable Wh / load (W) and minutes = hours × 60. Capacity, voltage and load must each be above zero, and the depth of discharge must be above 0 and at most 100; otherwise no result appears. The calculation assumes a constant load and a constant voltage, and it does not subtract inverter or converter losses, cable losses, or the capacity lost at high discharge currents or in cold weather. In practice, the real runtime is usually somewhat shorter than the figure shown, so leave a safety margin.

Worked example

A 12 V battery rated 100 Ah powers a 150 W load, and you only want to use half of its charge (DoD 50%). Total energy: 100 × 12 = 1,200 Wh. Usable energy: 1,200 × 50 / 100 = 600 Wh. Runtime: 600 / 150 = 4.00 h, which is 240 minutes. If the DoD were set to 80%, usable energy would be 960 Wh and runtime 6.4 hours, but this would wear the battery faster in many chemistries.

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Common mistakes

  • Entering the depth of discharge as a decimal such as 0.5. The field is a percentage, so 0.5 means half of one percent and the runtime comes out at minutes instead of hours; type 50.
  • Mixing up battery voltage and inverter output voltage. Use the battery's own voltage (12, 24, 48 V), not the 230 V or 120 V of the appliance.
  • Entering milliamp-hours (mAh) as Ah. A 5,000 mAh battery is 5 Ah, so divide by 1,000 first.
  • Using the appliance's maximum power rating as the load when it normally draws less, or using its average when it has high starting surges. Use the realistic continuous watts.
  • Ignoring conversion losses. Running a 230 V device from a battery through an inverter uses more power than the device label states, so the real runtime is shorter.

About this calculator

This calculator estimates how long a battery will run a given load from its capacity in amp-hours (Ah), voltage, load in watts, and usable depth of discharge (since most batteries shouldn't be fully drained). It converts capacity and voltage into usable watt-hours, then divides by the load to get runtime.

Off-grid solar users, RV and boat owners, and anyone sizing a backup battery for electronics use it to check whether a battery will last long enough before it needs recharging.

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Comments (1)

  • Editorial team

    If your load is rated in amps, multiply by the battery voltage first: 10 A on a 12 V system is 120 W. For anything running through an inverter, the battery supplies more than the plug-side wattage because the inverter wastes some, so enter a somewhat higher load to stay on the safe side. Treat the answer as the best case, and check the battery maker's guidance on depth of discharge. What kind of battery and load are you sizing?

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