In short. UPS battery runtime depends on the usable energy in the battery, the load, the inverter efficiency and the battery’s age and temperature. A quick estimate divides usable energy by load, but batteries deliver less energy at high discharge rates, so the maker’s discharge table at the actual load is the reliable figure. Design to the battery’s end-of-life capacity, keep it cool, and prove the runtime with a periodic load test.
Who this page is for: For IT, facility and hospital engineers checking or specifying UPS battery autonomy, and for anyone whose UPS runtime has fallen short.
Key facts
- Runtime (h) ≈ usable energy (kWh) × inverter efficiency ÷ load (kW).
- Usable energy = rated energy × depth of discharge × ageing factor × temperature factor.
- Design to end-of-life capacity, typically 80 to 85 percent of rated.
- Halving the load usually more than doubles runtime, because batteries deliver more energy at lower rates.
- Heat shortens lead-acid battery life sharply; keep batteries near their rated temperature.
- Runtime should cover generator start and transfer plus a failed first start.
The estimate
Worked example: end-of-life runtime (illustrative)
A battery string is rated 40 kWh at the 15-minute discharge rate; usable depth of discharge 0.8, end-of-life factor 0.85, temperature factor 1.0, inverter efficiency 0.95.
| Step | Working | Result |
|---|---|---|
| Usable energy at end of life | 40 × 0.8 × 0.85 × 1.0 | 27.2 kWh |
| Runtime at 100 kW | 27.2 × 0.95 ÷ 100 | About 0.26 h, 15.5 minutes |
| Runtime at 60 kW (simple scaling) | 27.2 × 0.95 ÷ 60 | About 26 minutes, and in practice usually a little more |
| Generator start, transfer and one failed start | Typically well under 2 minutes | Ample margin |
The same battery new would give more runtime; designing at end of life means the requirement is still met in year five. The maker’s constant-power discharge table at the actual load gives the real figure, because lead-acid batteries lose usable capacity at high discharge rates. See autonomy engineering.
Do not trust a battery’s float voltage as proof of runtime. An aged string can show normal voltage and collapse within minutes under load. Load-test it periodically and trend its internal resistance.
Factors to apply
| Factor | Typical consideration |
|---|---|
| Ageing | Design to end-of-life capacity, often 80 percent of rated |
| Temperature | Cold cuts capacity; heat cuts life |
| Discharge rate | High rates reduce usable energy |
| End voltage | A higher cut-off leaves less energy |
| Design margin | For uncertainty and growth |
What the runtime is for
- Bridging until the generator starts and takes the load, including a retry after a failed start.
- An orderly shutdown where no generator exists.
- Time for staff to respond to an alarm.
Long runtimes add cost, space and heat; a reliable generator usually adds more continuity. See UPS and critical power.
Chemistry notes
Valve-regulated lead-acid batteries are common and inexpensive but sensitive to heat and need regular testing. Lithium-ion batteries are lighter, last longer and tolerate deeper discharge, but need a proper battery management system and fire protection planning. See UPS batteries and monitoring.
Runtime worksheet
| Item | Value |
|---|---|
| Rated battery energy (kWh) and chemistry | |
| Depth of discharge, ageing and temperature factors | |
| Load (kW) and inverter efficiency | |
| Estimated runtime and maker’s table figure | |
| Last load test date and result |
Common mistakes
- Designing to new-battery capacity.
- Scaling runtime linearly with load.
- Hot battery rooms.
- No load tests.
What to send us
Send the UPS and battery model, the load and the runtime you need. WhatsApp 98852 01608 or use the enquiry form.
Evidence note. Factors are typical values for illustration; the battery maker’s discharge tables govern.
Questions engineers ask
How do I extend runtime?
Add battery strings or cabinets, reduce the load, or add a generator.
Why is runtime not proportional to load?
Batteries deliver less total energy at high discharge rates.
How often should I test capacity?
On a schedule set by the manufacturer and risk, often annually, with more frequent monitoring.
What is end-of-life capacity?
The point, often 80 percent of rated capacity, at which a battery should be replaced.
Figures are illustrative. Use manufacturer discharge data for design.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and Service Engineer, Reddy Generators. Last updated 8 November 2026.