Battery Backup (UPS) Calculator

About the Battery Backup (UPS) Calculator

Estimates how long a UPS or battery bank can power a load, from the battery voltage, amp-hour capacity, inverter efficiency and the load in watts.

How to use this calculator

1. Enter the battery voltage in volts, the battery capacity in amp-hours, and the efficiency as a decimal such as \(0.85\).
2. Enter the total load in watts, which is the power your devices use.
3. Use the calculator to estimate runtime in hours.
4. Compare the result to your backup needs, then adjust the battery or load if needed.

The formula explained

The formula estimates runtime in hours by dividing the battery's usable energy, \(V \cdot Ah \cdot eff\), by the load in watts, \(load_W\). A larger battery or higher efficiency increases runtime, while a larger load decreases it.

• V = battery voltage in volts
• Ah = battery capacity in amp-hours
• eff = efficiency as a decimal
• \(load_W\) = total load in watts
• hours = estimated backup runtime in hours

Step by step method

1. Multiply the battery voltage \(V\) by the capacity \(Ah\).
2. Multiply that result by the efficiency \(eff\).
3. Divide by the load \(load_W\) to get the estimated runtime in hours.

Worked example

Problem. A UPS uses a \(12\) volt battery with \(100\) amp-hours of capacity and an efficiency of \(0.85\). The connected devices draw \(170\) watts. Estimate the runtime.

1. Compute usable battery energy, \(12 \cdot 100 \cdot 0.85 = 1020\).
2. Divide by the load, \(1020 \div 170 = 6\).
3. The estimated runtime is \(6\) hours.

Tips and common mistakes

• Make sure the load is the total wattage of all connected devices, not just one device.
• Efficiency matters, so a lower \(eff\) means a shorter runtime even if the battery size stays the same.

Frequently asked questions