UPS Planning for Critical Factory and Office Loads: Size the Gap, Not the Outage

Many factory and office UPS problems start with a purchase made by kVA and habit: one large unit, an hour of battery "to be safe", and every socket in the building connected to it. Then a voltage dip still trips a line, or the batteries are found flat on the day the DG fails to start. A UPS is a bridge: it carries the loads that cannot ride through a disturbance until the DG or the grid takes over, or until those loads have shut down safely. Planning one starts with a schedule of those loads and the time each must be carried.

The UPS runtime calculator screens energy and runtime for a given load; this guide covers what to put into it and what it cannot decide.

Build the load and autonomy schedule

List every load that might go on the UPS, measured rather than taken from nameplates, and decide for each how long it must be carried and why.

Load and autonomy schedule checklist

• No-break loads — PLCs and control panels, servers and network equipment, critical instrumentation, controllers that lose position or data. These belong on the UPS.
• Short-break loads — motors, compressors, HVAC and general lighting restart when the DG picks up. Putting them on the UPS multiplies its size and its battery for little benefit.
• The awkward middle — drives on processes that must not stop. The answer may be a UPS on the drive's control supply only, a ride-through option offered by the drive manufacturer, or a UPS on the whole machine; decide machine by machine with the OEM.
• Life-safety systems — fire alarm panels, emergency lighting and similar systems have their own standby-power requirements under fire-safety rules. Do not treat a general-purpose UPS as their compliance route without confirming it with your fire consultant.

kW and kVA: two limits, both binding

A UPS has a kVA rating and a kW rating (kVA × its rated output power factor). The load must sit within both:

• Measure kW and kVA together. Adding up nameplates overstates some loads and misses the power factor of others.
• Modern IT power supplies can present a leading power factor at light load, and some UPS models derate for it. Check the manufacturer's output capability chart.
• Non-linear loads with a high crest factor, and inrush from transformers or motors, can push the UPS onto its static bypass. Keep heavy-inrush loads off the UPS or size for them.
• Leave headroom for growth and for redundancy, but avoid running a large unit lightly loaded for years: efficiency at low load and the capital tied up in batteries both suffer.

Topology and bypass

IEC 62040-3:2021 classifies UPS output performance. Three families matter in practice:

• Static bypass. Transfers the load to the bypass supply on overload or internal fault. A UPS left in bypass after an event protects nothing, so make bypass status a monitored alarm.
• Maintenance bypass. An external wrap-around bypass lets the UPS be isolated for service without shutting the load down. Specify it at the start; it is hard to add later.
• DG compatibility. On DG supply the frequency moves more than on the grid, and a UPS may refuse to synchronise its bypass and block transfers. The rectifier's input current distortion also loads the DG's alternator. Give the DG supplier the UPS input data (see DG expansion and synchronisation).
• Redundancy. Where a UPS fault would stop the process, two units in parallel or a modular UPS with an extra module (N+1) remove that single point of failure — provided the distribution downstream is designed so one fault does not take everything with it.

Battery autonomy: ageing, temperature and the OEM discharge tables

• Use the manufacturer's discharge tables. Runtime is read from the battery maker's constant-power tables (watts per cell or per block) to a stated end voltage at a stated temperature. A simple ampere-hour calculation overstates runtime at the high discharge rates a UPS draws.
• Size for end of life. Capacity falls with age. IEEE 1188, the recommended practice for maintaining, testing and replacing valve-regulated lead-acid batteries, treats capacity below 80% of the manufacturer's rating as a replacement point — so a bank that only just meets the autonomy when new will fall short long before it is replaced. Apply an ageing allowance and design for end-of-life capacity.
• Temperature works both ways. High room temperature shortens battery life (the manufacturer's life-versus-temperature curve shows by how much); low temperature reduces the capacity available. Air-condition the battery room and log its temperature.
• Chemistry. Valve-regulated lead-acid remains the lower-capital option. Lithium-ion (commonly lithium iron phosphate) costs more upfront, weighs less and generally tolerates temperature and cycling better, but brings its own fire-safety requirements (see lithium-ion UPS room fire safety). Compare both over the planned life, using each manufacturer's data.

Decision table: which UPS arrangement fits

Testing and maintenance that prove the autonomy

• Trend battery health — impedance or conductance against the commissioning baseline — instead of relying on float voltage alone.
• Run a controlled transfer test and, at intervals, a discharge test, so the autonomy is demonstrated rather than assumed. These can be written into an AMC.
• Keep bypass status, alarms and battery-room temperature on a monitored point.
• New input and output feeders and distribution boards for a UPS are alterations to the electrical installation: carried out by a contractor licensed for the work by the State Government, and approved or self-certified before connection under the CEA (Measures relating to Safety and Electric Supply) Regulations, 2023 (Regulations 31 and 45).

For the supply side around a UPS — DG sets, transformers and distribution — see Secured Engineers' electrical infrastructure service, or request a scope review with the schedule above.

Sources and how to verify

• International Electrotechnical Commission, IEC 62040-3:2021, Uninterruptible power systems (UPS) — Part 3: Method of specifying the performance and test requirements.
• IEEE, IEEE 1188, Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications.
• Central Electricity Authority, CEA (Measures relating to Safety and Electric Supply) Regulations, 2023, notified 8 June 2023: Regulations 31 and 45.

Confirm locally: your DG's measured start-and-transfer time, the UPS and battery manufacturers' data for the equipment actually selected, the fire-safety requirements for the battery room, and the voltage your state has notified for self-certification.