Industrial UPS Sizing for Factories

Industrial UPS sizing in one line #

Start with the loads that must remain energized, then check both UPS output limits:

Required kVA = critical kW ÷ load power factor

The selected UPS must have enough kW and kVA capacity, pass the manufacturer's short-time overload or motor-start curve, provide the required battery runtime, and meet the chosen redundancy policy. Runtime and N+1 are separate decisions; neither is created by adding an unexplained safety factor.

Calculate UPS capacity for 39 kW at 0.95 PF →

This guide owns the factory selection process. The calculator owns instant kVA results, while the UPS runtime guide explains battery-energy estimates.

Best for: stationary AC UPS planning for a defined factory critical-load bus, including base kW/kVA, load steps, runtime and modular redundancy decisions.

Not suitable for: choosing a UPS from whole-plant connected load, DC UPS, rotary UPS, site-wide generator sizing, battery purchase from simple Wh arithmetic, or final protection/bypass/earthing design without the exact manufacturer data and applicable code/standard review.

Step 1: define the protected load #

Do not begin with the whole facility demand. Build a UPS load schedule containing only equipment that must operate through the outage or until the generator accepts load.

For each load, record:

• measured or manufacturer-rated input kW and power factor;
• normal, starting and temporary overload states;
• whether loads can start simultaneously;
• required ride-through time and shutdown sequence;
• whether the load is already redundant.

An illustrative factory schedule might contain 28 kW of PLCs, controls, servers, safety systems and sensors, plus 11 kW of production monitoring and communications. If both groups require backup, the UPS design load is 39 kW. This is an example inventory, not a default load allowance.

Step 2: convert critical kW to kVA #

For 39 kW at a measured or specified input PF of 0.95:

kVA = 39 kW ÷ 0.95 = 41.1 kVA

Now compare 39 kW and 41.1 kVA with the UPS manufacturer's output ratings. A UPS can reach its kW limit before its kVA limit, especially when the load PF differs from the rating condition.

If the project reserves 20% for documented near-term growth, the planning screen becomes:

• kW screen: 39 × 1.20 = 46.8 kW
• kVA screen: 41.1 × 1.20 = 49.3 kVA

A nominal 50 kVA frame is only a candidate. It still has to satisfy its published kW rating, operating temperature, bypass arrangement and overload curve. Eaton's sizing guidance recommends accounting for future expansion and checking both real and apparent power; its published 15% growth allowance is a planning recommendation, not a universal code rule.

Step 3: check starting and nonlinear loads #

Steady-state kVA does not prove that a factory UPS can start a motor, energize a transformer or support a large rectifier step. For every load that can change state while on battery:

1. obtain starting or step-load kVA and duration from the equipment supplier;
2. identify the worst credible concurrent event;
3. compare it with the exact UPS model's overload-versus-time curve;
4. if it fails, sequence the load, use a VFD or soft starter where appropriate, move it off the UPS bus, or select a different frame.

Do not assume a generic percentage or horsepower cutoff. UPS overload capability varies by topology, operating mode and manufacturer.

Step 4: set runtime independently #

Runtime begins with an operating requirement, such as controlled shutdown or the maximum generator start-and-transfer sequence. It is not derived from UPS kVA.

Define the load profile during the outage, the required minutes, battery chemistry, string voltage, temperature range and end-of-life criterion. Then use the UPS Runtime Calculator for an energy screen and verify the result against the selected manufacturer's battery and runtime tables. High-rate battery discharge is nonlinear, so simple Wh arithmetic is not a purchase specification.

For battery Ah and string layout, continue to the UPS battery sizing guide. For generator coordination, use the Generator UPS Calculator.

Step 5: decide whether N+1 is required #

In an N+1 arrangement, N modules can support the design load and one additional module permits a single module outage. Check the load after one module is unavailable; do not count the same reserve once as growth margin and again as redundancy.

Use the UPS Redundancy Calculator to screen module count and post-failure loading. The final topology also depends on bypass, maintenance isolation, battery arrangement and whether downstream distribution contains a single point of failure.

Factory UPS selection worksheet #

What UPS sizes are available in kVA? #

There is no single international “standard UPS size” ladder. Available kVA frames and their corresponding kW ratings vary by manufacturer, topology, voltage, module family, region and parallel configuration. Treat search results such as 10, 20, 40 or 50 kVA as catalog labels, not guaranteed steps. First calculate the required kW and kVA; then compare current eligible product families and verify both ratings at the stated conditions.

Common sizing mistakes #

1. Using total plant connected load: only the protected operating sequence belongs in the UPS schedule.
2. Checking kVA but not kW: both output limits apply.
3. Treating a margin as motor-start capacity: the OEM overload curve governs the transient check.
4. Converting kVA directly into runtime: battery energy and discharge performance are separate.
5. Calling spare capacity N+1: redundancy is demonstrated after a module is unavailable.
6. Selecting from nominal size alone: temperature, bypass, harmonics, fault behavior and manufacturer conditions can change the result.

Technical sources #

• Eaton UPS sizing guide — separate kW/kVA checks, growth planning and N+1 terminology.
• IEC 62040-3:2021 — performance and test requirements for complete electronic AC UPS systems; confirm the current edition at specification time.
• IEC 62040-1:2017 with amendments — UPS safety scope for movable, stationary, fixed and built-in systems; the IEC page identifies Amendments 1:2021 and 2:2022.

FAQ #

Next step #

Freeze the protected load schedule, screen the frame in the UPS Capacity Calculator, then verify runtime and redundancy separately before comparing vendor models. Use the UPS Calculator hub to keep load, capacity, runtime, battery, generator and redundancy steps separate.