UPS Selection Checklist: 6 Things to Check Before You Buy

“Can a 10 kVA UPS support 6 kW of servers for two hours?”

It might, but the kVA rating alone cannot answer the question. The UPS must have enough output capacity to carry the load, and its batteries must provide the required runtime. The site’s electrical supply, installation conditions and any short-term power peaks matter too.

Quick answer: A 10 kVA UPS can be a candidate for a 6 kW load if its rated kW output and overload capability are sufficient. Two hours of backup requires a separately sized battery system. Check the following six points before choosing a model.

1. Capacity: check kW and kVA

UPS capacity is usually shown in kVA, but the datasheet should also state its rated output in kW. Both limits matter.

For example, a 10 kVA / 8 kW UPS and a 10 kVA / 10 kW UPS have the same apparent-power rating, but different real-power ratings. A 6 kW load uses 75% of the first model’s rated kW output.

Start by listing everything that will be connected to the UPS. For an existing site, measure operating power during typical and busy periods where possible. A server fitted with an 800 W power supply does not necessarily consume 800 W continuously. Keep its nameplate rating for maximum-demand checks, but do not mistake it for measured consumption. Our article on rated, actual and peak UPS load power explains these figures in more detail.

If the load consumes 4 kW at a measured power factor of 0.9, its apparent power is approximately:

4 kW ÷ 0.9 = 4.44 kVA

Now compare 4 kW with the UPS’s kW rating and 4.44 kVA with its kVA rating. The load’s power factor and the UPS’s rated kW-to-kVA ratio describe different things; do not interchange them.

Also allow for planned growth. Some projects use 80% of rated capacity as a planning limit, but it is not a universal requirement. State the expected growth and check the proposed model’s actual ratings rather than applying several unexplained margins.

2. UPS type: match the protection to the equipment

For equipment that cannot tolerate a brief supply interruption, online double-conversion is often considered. Its normal transfer to battery has no interruption caused by changing the load from a utility path to an inverter path. That does not mean every transfer to bypass, every operating mode or every fault condition is interruption-free. Check the specifications of the exact model.

High-frequency, low-frequency and modular describe other design choices. Compare models using their rated output, overload performance, expected operating efficiency, service access and expansion needs. TAFENG provides both high-frequency UPS and low-frequency UPS options; the appropriate choice depends on the project requirements.

3. Batteries: specify runtime at a stated load

UPS output capacity answers how much load the unit can support. Battery sizing answers how long it can support that load.

A request for “two hours of backup” must state the load during those two hours. At 6 kW, the energy delivered to the load is:

6 kW × 2 hours = 12 kWh

This 12 kWh is not the battery capacity to order. The battery design must account for UPS losses, battery discharge characteristics, end-of-discharge voltage, temperature and aging. Confirm runtime using the proposed UPS and battery configuration’s discharge or runtime data.

Also check whether the model uses internal batteries, accepts external battery packs or needs a separate battery cabinet. For extended runtime, verify the permitted battery voltage and configuration, charger capacity and expected recharge time. A battery bank that meets the first outage requirement may still be unsuitable if it cannot recharge quickly enough for the site’s operating needs.

4. Input and output: match the site’s electrical supply

UPS systems are available with single-phase input and output, three-phase input and single-phase output, or three-phase input and output. Do not choose among them by kVA rating alone.

Before selecting a model, confirm the available input voltage and phases, the required output voltage and phases, upstream breaker and cable capacity, grounding and neutral arrangements, and the bypass supply.

A three-phase UPS can supply appropriately distributed single-phase loads where its design permits, but each phase still has a limit. Check phase balance and the most heavily loaded phase. If the system has a bypass path, confirm that its source and wiring can support the load when the UPS transfers to bypass.

5. Installation: check the room as well as the UPS

A specification that says only “install in a utility room” leaves important questions unanswered.

For a rack-mounted UPS, check cabinet depth, rail compatibility, weight capacity, airflow and maintenance access. For a floor-standing UPS or external battery cabinet, check the delivery route, floor loading, ventilation and space needed to replace batteries.

Consider what happens to the room during an outage. If cooling stops while the UPS continues to power the load, temperature may rise before the requested battery runtime ends. Battery performance and service life are especially sensitive to installation conditions.

Outdoor systems need equipment or an enclosure suited to the site’s temperature, moisture, dust and corrosion exposure. Select the enclosure and thermal design for the actual location rather than assuming one protection rating fits every site.

6. Load behaviour: check startup and fault conditions

Steady operating power does not describe every demand a UPS will face.

Motors, pumps and compressors may draw much more current while starting. Multiple IT devices switched on together can also create a short peak. Ask for the largest expected starting current, how long it lasts and whether several loads may start at once. Compare those figures with the UPS’s overload and peak-current specifications, including operation on batteries.

For dual-power equipment, check what happens when one supply path fails. The remaining path may have to carry more load than it does in normal operation.

An overload may cause a UPS to transfer to bypass if bypass is available and the transfer conditions are met. If bypass is unavailable, the outcome may be different. Review normal, battery and fault conditions before approving the design.

What to send when requesting a UPS recommendation

A request that says only “10 kVA UPS, two-hour backup” leaves too much open to interpretation. With these details, the UPS model and battery configuration can be proposed together and checked against the site.

Send your project requirements to TAFENG for a UPS and battery recommendation.