UPS Runtime Calculator
Estimate how many minutes a UPS or DC battery bank can carry its load before the batteries are exhausted. Enter the load, the battery voltage and amp-hour capacity, and the tool works out the usable energy and the runtime in both minutes and hours-and-minutes.
Enter Values
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How to use this calculator
- Enter the load in watts (the total power the UPS is supporting) and the battery voltage and capacity (Ah) of the bank.
- Set the number of parallel battery strings, inverter efficiency and usable depth of discharge, or leave them at the defaults (1 string, 90% efficiency, 100% DoD).
- Read the estimated runtime in minutes and as hours:minutes, plus the usable battery energy in watt-hours.
How it works
Usable energy in watt-hours is battery voltage x capacity (Ah) x number of strings x inverter efficiency/100 x depth-of-discharge/100. Runtime in minutes is that usable energy divided by the load in watts, multiplied by 60. Higher efficiency, more strings or a deeper allowable discharge all increase runtime, while a heavier load reduces it.
Worked example
Worked example. A 500 W load runs from a 48 V, 100 Ah battery string at 90% inverter efficiency and 100% depth of discharge. Usable energy = 48 x 100 x 1 x 0.9 x 1.0 = 4,320 Wh. Runtime = 4,320 / 500 x 60 = 518.4 minutes, or about 8 h 38 min.
Common mistakes
- Assuming the full rated Ah is available at UPS discharge rates — the Peukert effect means fast discharge delivers noticeably fewer amp-hours than the slow rating.
- Ignoring depth of discharge — flooded lead-acid banks are often limited to 50% DoD to protect cell life, which halves the usable energy versus a 100% figure.
- Forgetting that batteries lose capacity with age, temperature and cycle count, so a set several years old will not match its nameplate.
Frequently asked questions
Why is my real UPS runtime shorter than this estimate?
This is an ideal calculation. Real batteries suffer the Peukert effect at high discharge rates, lose capacity as they age and in the cold, and the inverter draws a little extra for its own control circuitry. Treat the result as an upper bound and confirm with a load-bank test.
What depth of discharge should I use?
It depends on chemistry. Lithium (LiFePO4) banks tolerate 80-100% DoD, while lead-acid is usually limited to around 50% to preserve cycle life. Use the value your battery manufacturer specifies for the number of cycles you want.
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