UPS runtime and battery sizing: a practical guide
UPS runtime depends on the real load, the battery's capacity on the day of the outage, its temperature and its age. Size the battery for the end of its life, not the day it goes in. CSB advises replacing a string at 80% of its rated capacity [1]. A string that only just meets the target when new falls short long before it is due.
Key facts
- Runtime depends on: measured load in kW, battery capacity, temperature and age
- End of battery life: replace at 80% of rated capacity or before (CSB)[1]
- Temperature: CSB rates operation from 5 to 35 °C, with an average below 25 °C[2]
- Cold: Tripp Lite notes reduced runtime below 10 °C[3]
- Lead-acid service life: generally 3 to 5 years, against 10 to 15 for lithium-ion (Eaton)[4]
Decide what the runtime is for
Runtime has a job, and the job sets the number:
| Site | What the battery must cover |
|---|---|
| Generator and transfer switch | The generator start and the transfer, plus a margin for a second start attempt |
| No generator | An orderly shutdown of the systems that matter, in the order they must stop |
| Network closet on a building generator | The gap until the building’s generator picks up the closet’s circuit |
Write the figure down with the reason next to it. Everything else is sized against it.
Measure the load, not the nameplate
Nameplate ratings are maximums. The load that matters is what the UPS carries, in kilowatts, read from its display or a power meter over a busy period, plus the growth expected over the life of the string. A battery sized from nameplates is usually far larger than needed. One sized from a quiet afternoon reading can be too small.
Read the runtime chart at that load
UPS and battery makers publish runtime charts or calculators for each battery configuration. Runtime does not scale in a straight line with load: halving the load usually more than doubles the runtime. Use the chart for the exact UPS model and battery configuration, at the measured load.
Add the margins
- Age: size for the runtime the string gives near replacement, at 80% of its rating [1].
- Heat: warm batteries age faster. Tripp Lite’s manual halves the expected battery life for each 10 °C above 25 °C [5].
- Cold: cold batteries deliver less; runtime drops below 10 °C [3].
- Growth: the load in the string’s last year, not its first.
- The low-battery warning: the last minutes are not usable runtime. A SmartOnline unit warns when less than 3 minutes remain [6], and your shutdown scripts should finish before then.
Formal methods
Large stationary strings are sized to IEEE 485 for lead-acid and IEEE 1184 for UPS duty. For most server rooms, the manufacturer’s chart with the margins above gives the same answer.
Standards cited
- IEEE 485
- IEEE 1184 (named as the formal references
- not summarised here)
Common questions
How do I calculate UPS runtime?
Measure the load in kW, read the manufacturer's runtime chart for your exact UPS and battery configuration at that load, then allow for battery age and temperature.
Why is my UPS runtime shorter than the chart says?
Usually the batteries are older or warmer than the chart assumes, the load has grown, or one weak block is pulling down the string. A battery test and a load reading show which.
How much margin should a UPS battery have?
Enough to meet the runtime target when the string has aged to its replacement point. CSB puts that point at 80% of rated capacity or before [1].
Do lithium-ion batteries change the sizing?
They change the life and the weight more than the method. Eaton gives lithium-ion UPS batteries 10 to 15 years, against 3 to 5 for lead acid [4], so fewer replacements fall inside the UPS's life. Size from the lithium system's own runtime data.
Sources
- CSB Energy Technology, VRLA Battery User Manual (Operation and Handling, Rev Dec 2024), failure modes table
- CSB Energy Technology, VRLA Battery User Manual (Operation and Handling, Rev Dec 2024), operating temperature
- Eaton (Tripp Lite), Tripp Lite by Eaton SmartOnline SU750-SU3000RTXLCD2U Owner's Manual 614-21325 Rev 05, section 5.1 (PDF p. 37)
- Eaton, Lithium-ion UPS FAQ, FAQ
- Eaton (Tripp Lite), Tripp Lite by Eaton SmartOnline SU750-SU3000RTXLCD2U Owner's Manual 614-21325 Rev 05, section 5.1, Equipment Care (PDF p. 37)
- Eaton (Tripp Lite), Tripp Lite by Eaton SmartOnline SU750-SU3000RTXLCD2U Owner's Manual 614-21325 Rev 05, section 4.7 (PDF p. 36)
Updated