Uninterruptible power supply systems, sized for the real load.

Which UPS capacity fits your load?

Start with the real power in kW, not the nameplate VA. Divide by the output power factor (typically 0.9 for online UPS) and add a design margin of 20–25%. Then check the largest motor or compressor starting current, because a 5 kW motor can draw 25–35 kW for a few seconds.

The table below is a starting point for common loads. A 10 kVA UPS does not mean 10 kW; at 0.9 PF it delivers 9 kW, and at 80% loading a prudent design uses about 7.2 kW continuously.

The capacity table is a first-pass guide. Motor starting, harmonics, altitude, and ambient temperature can change the required size.

How long should the UPS battery last?

Runtime is a project decision, not a UPS feature. Five minutes is enough to ride through a generator start or shut down a server. Thirty minutes covers a typical utility switching event. Two to eight hours is for sites without a generator or for telecom and security loads that must continue through the night.

The battery bank is sized with the DC bus voltage, the inverter efficiency, the usable depth of discharge, and the discharge-rate curve. A simple Ah calculation underestimates the bank at short runtimes because the battery delivers less than its 20-hour rating at high current.

These figures are indicative and must be recalculated against the selected battery’s discharge curve and the UPS charger limit. Use the sizing calculator for a first estimate.

Single-phase or three-phase UPS?

Single-phase UPS systems are used where the load is single-phase or where the site supply is single-phase: a branch office, a clinic, a small server rack, a security room. Three-phase UPS systems are used when the load is three-phase, when the supply is three-phase, or when the capacity is above about 20 kVA.

A three-phase UPS can also supply single-phase loads through its neutral, but the load must be balanced across phases. If most of the load is single-phase and distributed, a three-phase UPS with a large neutral and independent phase monitoring is the better choice.

• Single-phase online: 1–10 kVA, tower or rack.
• Three-phase input / single-phase output: 10–20 kVA, for server rooms and clinics.
• Three-phase input / three-phase output: 10–200 kVA, for facilities and industrial plants.
• Modular three-phase: 90–300 kW, for data centers with N+1.

What should be in a UPS quotation?

A useful quotation is more than a model and a price. It should state the rated kVA and kW, the output power factor, the input voltage window, the battery configuration and runtime basis, the dimensions and weight, the communication options, the protection rating, and the documentation package.

For a project, also ask for the single-line diagram, the breaker and cable sizing, the battery cabinet layout, the commissioning checklist, and the lead time for the exact configuration. If the site is in a hot or high-altitude location, ask for the derating table.

• Load list and diversity factor used for the calculation
• UPS model, kVA, kW, output PF, and overload curve
• Battery model, string configuration, and runtime basis
• Input/output terminals, breaker size, and cable size
• Dimensions, weight, heat rejection, and clearance
• Monitoring, SNMP, dry contacts, and MODBUS options
• Certification and test documentation available by model
• Packing, shipping terms, and lead time

Which standards and documentation apply?

Online UPS systems are commonly specified against IEC 62040-1 (safety), IEC 62040-2 (EMC), and IEC 62040-3 (performance and test methods). The modular and large-frame units also reference IEC 60950-1 where applicable. Battery blocks are specified with their own cycle-life and transport documentation.

For lithium modules, ask for UN38.3, MSDS, CE, RoHS, and the BMS communication protocol. For a tender, confirm whether the certificate is issued to the manufacturer, the model, or the factory, and whether a model-specific test report is available.