What runtime actually means
Runtime (or autonomy) is how long a UPS can power your load on battery alone. It is not fixed by the UPS size — it depends on the battery capacity and the load. A 10 kVA UPS at 20% load will run far longer than the same UPS at 90% load. Runtime curves are non-linear: doubling the battery roughly doubles runtime, but halving the load more than doubles it.
Typical runtimes
Most UPS with internal batteries provide 5–15 minutes at full load — enough to ride through brief outages and trigger a graceful shutdown. Extended Runtime (ER) models and external battery cabinets push this to 30 minutes, an hour or more. For sites that need to ride out extended outages without a generator, multiple battery strings or lithium-ion banks can deliver hours of autonomy.
How to extend runtime
- Add external battery cabinets or extra battery strings.
- Choose an Extended Runtime (ER) model.
- Reduce the load — protect only what truly needs to stay up.
- Consider lithium-ion batteries, which pack more runtime into less space and last 8–10 years vs 5 for VRLA.
- Pair the UPS with shutdown software so non-essential systems power down quickly, leaving more battery for critical loads.
Runtime vs generator
If you need more than ~30 minutes of cover, a generator is usually more economical than stacking battery cabinets. The UPS holds the load for the 10–30 seconds the generator needs to start and stabilise, then the generator sustains the site for hours. The two are designed to work together.
Frequently asked questions
How long does a UPS battery last during a power cut?
Typically 5–15 minutes at full load on internal batteries, extendable to an hour or more with external battery cabinets or lithium-ion banks.
How is UPS runtime calculated?
Runtime depends on battery capacity (Ah) and the actual load (kW). It is non-linear — lower loads give disproportionately longer runtimes. Use a runtime calculator for an accurate estimate.
Can I add more batteries to a UPS later?
Yes, most UPS accept external battery cabinets or additional strings. Some require a dedicated charging upgrade for large battery banks.
Are lithium-ion UPS batteries worth it?
For long runtime or space-constrained sites, yes — they last 8–10 years, charge faster, weigh less and pack more runtime into a smaller footprint than VRLA.