The most common UPS question is “how long will it run on battery?” The honest answer is “it depends” – and understanding what it depends on lets you size the system correctly instead of guessing.

What runtime depends on

• Real load (in watts / kW), not the UPS nameplate rating. A lightly loaded UPS runs longer.
• Battery bank energy – the Ah rating, the number of blocks and the DC bus voltage.
• Battery chemistry – VRLA or lithium LFP behave differently under discharge.
• Inverter efficiency and conversion losses.
• Temperature and battery age – high temperature and ageing both reduce usable capacity.

Because runtime falls non-linearly as load rises, a UPS at 50% load runs much longer than at full load – not just twice as long.

A practical sizing process

1. Measure the real load. Add up the actual power draw of the connected equipment, with a margin for growth.
2. Set a backup-time target. Decide whether you need minutes to ride through short cuts and start a generator, or longer autonomy.
3. Choose the battery type. See VRLA vs lithium for the trade-offs.
4. Decide internal, built-in module or external batteries. Some compact UPS systems use internal batteries or built-in battery modules; longer-runtime projects normally use external battery cabinets.
5. Add margin for temperature and ageing. Size for end-of-life capacity and the real ambient temperature, not ideal lab conditions.
6. Confirm against the exact UPS model. Final battery quantity, cabinet design, charger setting and runtime reference should be validated against the UPS series, site load and installation conditions.

Internal, built-in module and external batteries

Entry-level units such as the line-interactive LT1E use internal batteries for standard short backup. Single-phase online systems such as TM11E and RM11E may use internal or external battery configurations by model. Larger online UPS projects normally use external battery cabinets sized to the target runtime, including TM33E, RM33E, TM33-ET and TM66E platforms.

VRLA is common across many ATENCO UPS ranges. Lithium / LFP support is available only on selected platforms and project-confirmed configurations, such as RM11E PRO, TM33E 50–200 kVA, TM66E Modular 40–600 kVA and TM66E Large Modular 400–1200 kVA. Exact battery cabinet models, quantities, Ah ratings, charger settings and BMS requirements are selected per project.

Runtime figures depend on load, battery configuration, temperature, battery age and UPS efficiency. ATENCO reviews runtime by project; request a runtime reference with your UPS model, load, battery preference, backup-time target and site temperature.

Get a sizing reference

Send the UPS model (or intended series), the real load, the battery type, the battery quantity if known, the runtime target and the site temperature using the Request a Document form, or shortlist a series with the UPS Selector. ATENCO will review the information and provide a runtime reference or next-step clarification where applicable.