TechCalc
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Power · Batteries and power banks

Battery runtime calculator

Estimate how long a power bank, battery or portable power station could run a device at a given power draw.

✓ Free calculator✓ No sign-up✓ Instant results

Battery settings

Enter capacity as mAh or Wh.

Choose units below.
Only used for mAh; ignored for Wh.
Example, not a measured device specification.
Accounts for discharge limits and operating conditions.
Your settings are saved only in this browser on this device.

How does it work?

If capacity is given in mAh, convert it to Wh first: mAh × V / 1000. If capacity is already in Wh, voltage is not used.

Runtime = Wh × conversion efficiency × usable capacity / device power in W. This simplified model does not include discharge curves, voltage sag, temperature or transient loads.

Important: A power bank’s mAh rating typically refers to internal cell voltage, not the 5V USB output. Voltage matters.

Frequently asked questions

Is a 20,000 mAh power bank rated at 5 V?

Usually not. The mAh specification typically refers to the internal battery cell voltage.

Why might actual runtime be shorter?

Conversion losses, temperature, ageing and variable device loads all affect runtime.

Equipment related to this calculation

Practical, non-promotional buying considerations. Future partner recommendations will be clearly disclosed.

USB-C power bank

Check energy in Wh, charging protocol and output power, not only the advertised mAh.

Portable power station

Compare usable energy, inverter efficiency and supported device loads.

Frequently asked questions

Why is a 20,000 mAh power bank not 20,000 mAh at USB 5 V?

The mAh rating is tied to the cell voltage, commonly around 3.7 V. Voltage conversion also causes losses.

Is the runtime exact?

No. The estimate assumes roughly constant load. Temperature, battery age, losses, and varying load affect the actual runtime.