Battery Backup Calculator

Estimate how long your battery will last during a power outage. Input your battery's specifications and the total power consumption of your devices to calculate the expected runtime. Perfect for planning with UPS systems, power banks, and solar setups. Explore more tools on free electronics calculators on CalculatorBolt.

Calculate Battery Runtime

Required if capacity is in mAh/Ah
Auto-calculated from device table or manual input

Add Devices

Device Name Power (Watts) Action

For lead-acid or solar batteries. Li-ion is typically 100%.

How It Works

The calculator first determines the total energy stored in your battery in Watt-hours (Wh). It then applies an efficiency factor to account for energy lost during conversion (from DC to AC). Finally, it divides this usable energy by your devices' total power draw (in Watts) to find the estimated runtime in hours.

Inputs Explained

  • Battery Capacity: The total charge your battery can hold, found on its label.
  • Battery Voltage: The voltage of the battery (e.g., 12V for a car battery, 3.7V for a power bank cell). Required if capacity is in mAh or Ah.
  • Total Device Load: The combined power consumption of all devices you want to run, measured in Watts.
  • Inverter Efficiency: The percentage of battery power that is successfully converted to usable AC power. 90% is a common average.

Example

A 12V battery with a 100Ah capacity is used to power a 100W light bulb via an inverter with 90% efficiency. Total Energy = 100Ah × 12V = 1200 Wh. Usable Energy = 1200 Wh × 0.90 = 1080 Wh. Runtime = 1080 Wh / 100W = 10.8 hours. The estimated backup time is 10 hours and 48 minutes.

Tips & Notes

  • Check your device's power adapter or manual for its wattage rating.
  • Real-world runtime can be shorter due to battery age, temperature, and fluctuations in device power draw.
  • For UPS systems, use the 'VA' rating as a rough guide for wattage, but check the manual for the specific Watt rating if available (Watts = VA × Power Factor).

FAQs

Watts measure real power consumed, while VA measure "apparent" power. For many devices, they are similar, but for motors and older electronics, VA can be higher than Watts. Your actual load is in Watts.

Look for a label on the device or its power adapter. It will list the input voltage (V) and amperage (A). Wattage is calculated as Voltage × Amperage.

A quality pure sine wave inverter typically has an efficiency of 85-95%. Lower quality modified sine wave inverters may be less efficient. 90% is a safe estimate for planning.

Battery capacity decreases over time and with temperature. Also, devices with motors (like refrigerators) can have a high starting surge power that isn't reflected in their average wattage.

Disclaimer

Important: This calculator provides an estimate of battery backup time based on the inputs provided. Actual runtime may vary due to factors like battery health, temperature, device power fluctuations, and inverter quality. This tool is for planning purposes only.

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Author: CalculatorBolt Editorial Team
Reviewed by: Electrical Engineer
Published: • Updated: