UPS runtime and battery sizing: what to ask before you buy

Define what must happen before battery support ends

Write the operating outcome first: bridge a power-source transition, finish a controlled shutdown, or support a specified period of operation. Then identify the systems that must remain available for that outcome. A shutdown plan that depends on network equipment outside the protected scope has an unresolved dependency, even if the servers have adequate battery support.

Separate the required service interval from the design allowance that your engineer selects. Record who owns each assumption. This keeps an ambitious runtime request from becoming an unexplained number that different suppliers satisfy in different ways. For an existing installation, use the current load schedule rather than the workload expected when the UPS was originally installed.

Keep UPS output capacity separate from stored energy

Eaton distinguishes how much equipment a UPS can support from how long batteries can support it. Its battery handbook also explains that adding batteries may extend runtime without increasing the UPS output capacity. Both limits matter; solving one does not automatically solve the other.

Use two lines in your brief: the required output load in kW and kVA, and the required runtime at that load. Ask the supplier to identify the UPS rating and battery arrangement that support those lines. Avoid shorthand such as a 30-minute UPS without a corresponding load and a specific battery configuration.

Sources: Eaton UPS sizing guide ↗ · Eaton three-phase UPS battery handbook ↗

Read a runtime curve as a configuration-specific claim

Manufacturer runtime tools are a better starting point than extrapolating from a battery nameplate. Eaton publishes product-specific graphs with optional battery-pack configurations. Select the exact UPS variant and battery arrangement, then find the proposed operating load and its associated runtime. Save the selected inputs with the output.

For a hypothetical procurement review, suppose Vendor A states 15 minutes but supplies no load value, while Vendor B provides 12 minutes at the specified design load. Those claims do not demonstrate that A is better. Ask A to regenerate its result on the same basis before comparing. These example times illustrate evidence quality; they are not predictions for a product.

If your exact load lies between published points, ask for an approved calculation rather than inventing an interpolation method. Where a supplier uses a proprietary sizing tool, request a dated report containing the inputs, selected equipment and the conditions under which the stated result applies.

Sources: Eaton UPS battery runtime graphs ↗

Use watt-hours as a sense check, not a battery bill of materials

A hypothetical constant 40 kW AC load operating for 15 minutes receives 10 kWh of energy: 40 × 15/60. This is energy delivered to the load. It is not a recommendation to purchase a 10 kWh battery, and it does not tell you how many battery cabinets are required.

Battery capacity is described under specified conditions, including discharge rate, temperature, state of charge and end voltage. The Eaton battery handbook discusses these dependencies and the effects of aging. Your supplier must account for the chosen equipment and conditions in its runtime calculation. An arithmetic sense check is useful for spotting mismatched units, but it cannot replace that evidence.

Sources: Eaton three-phase UPS battery handbook ↗

Send a battery sizing brief that removes ambiguity

The following template is deliberately organized around questions a purchasing team can answer or assign. Leave an unknown field marked unresolved and name the party who will establish it. Do not let a blank temperature or end-of-life assumption become an undocumented default.

Compare alternatives without confusing a longer quote with a better design

Organize each offered battery package into three views: the performance claim, the installation scope and the ownership plan. In the first, record supported load, duration and assumptions. In the second, ask for the space, access and associated work included in the offer. In the third, ask about inspection, monitoring, replacement and disposal arrangements.

If one proposal uses a different chemistry or a different UPS frame, request a complete explanation of the changed configuration. Do not carry forward the previous runtime result or assume that the same cabinet count means the same service. A useful commercial question is: which obligations and assumptions change if we choose this alternative?

Keep recharge readiness explicit. Ask whether the quoted recovery condition assumes a particular load or available source capacity, and how the supplier would assess repeated outages. This is a request for project-specific evidence, not a universal recharge-time promise.

Carry the runtime requirement into commissioning and maintenance

Agree how the delivered configuration and claimed performance will be documented before accepting the purchase. Any discharge testing should be planned and performed by qualified personnel under an approved procedure; this article is a purchasing checklist, not instructions to interrupt an operating power system.

Store the final runtime report beside the equipment list and acceptance record. Reopen the calculation when the protected load, battery configuration or operating conditions change. A screenshot from the original sale is most useful when future reviewers can reconstruct exactly what it described.

• Plan future battery replacement ↗

Common questions