What a UPS Does, and How the Main Types Differ

On this page

1. 01Before you start
2. 02What a UPS actually does
3. 03Where it sits in the project
4. 04The three main topologies
5. 05Comparison and specs that matter
6. 06Capacity: VA, kVA, watts, kW and power factor
7. 07How to size capacity
8. 08Battery runtime and autonomy
9. 09VRLA vs lithium-ion
10. 10Installation: floor loading, cooling, bypass
11. 11Critical features
12. 12Applications
13. 13UPS vs stabilizer vs generator
14. 14Check that it works
15. 15Common problems and how to fix them
16. 16Frequently asked questions
17. 17Talk to PSS about UPS systems

A power outage does more than turn off the lights. It can interrupt operations, corrupt databases, drop network links, stop CCTV recording, disable access control and halt industrial processes.

An Uninterruptible Power Supply (UPS) maintains continuity when utility power fails, can improve power quality, support controlled shutdowns and bridge the gap until a standby generator starts. A desktop standby unit is not the same product as a line-interactive rack UPS or an online double-conversion system for a data room, PLC panel or critical security stack. Topology, load calculation, power factor, battery autonomy, distribution, cooling and maintenance decide whether the system actually protects the site.

Before you start

• Measured critical load in kW — and power factor?
• Required runtime at that load — and at future load?
• Which loads must ride through vs tolerate ~10 s generator transfer?
• Topology fixed by client standard or open?
• Battery chemistry — is the battery room air-conditioned?
• Footprint, floor loading and heat rejection available?
• Generator kVA, rectifier type, input THDi limit?
• Neutral earthing — is 4-pole transfer required?
• Monitoring: SNMP, Modbus or dry contacts?
• Runtime evidence: witnessed test or factory report?

What a UPS actually does

A UPS is not simply a large battery. It combines power conversion, energy storage, control and monitoring to support connected equipment when the normal supply is interrupted or unsuitable.

Immediate backup. On utility failure the UPS uses stored battery energy. Standby and line-interactive designs normally have a brief transfer interval to battery; online double-conversion normally keeps feeding the load through the inverter with no normal transfer interruption. That continuity can allow safe server shutdown, keep networks and CCTV alive, or wait for a generator.

Power conditioning. Depending on design, a UPS can reduce undervoltage/sags, overvoltage/swells, selected transients, limited frequency variation and some noise/distortion. Online double-conversion generally isolates the load more strongly because the inverter supplies it continuously. No UPS provides absolute immunity to lightning, severe surges, grounding faults, harmonics or every PQ event — additional engineered protection may still be required.

Monitoring and shutdown. Commercial units may offer battery status, overload alarms, event logs, remote communications and software that starts an orderly shutdown before the battery hits its low-voltage cut-off. In unattended IT rooms those features can matter as much as the kVA label.

Four standards frame a professional specification: IEC 62040-1 (safety), IEC 62040-2 (EMC), IEC 62040-3 (performance/test) and IEC 62040-4 (environmental). IEC 62040-3 classifies output as input dependency (VFD / VI / VFI), waveform (SS THD ≤ 8%, SX ≤ 12%, SY non-sinusoidal) and dynamic class (1 = 0 ms, 2 = 1 ms, 3 = 10 ms). On that grid, double conversion is typically VFI-SS-11, line-interactive VI-SS-31, passive standby VFD-SY-33. A datasheet that only says “online” does not prove a true VFI design versus eco-mode that drops toward VFD behaviour on the first dip.

Where it sits in the project

Utility → main board → ATS with generator → UPS → critical-loads board. Place the UPS downstream of the ATS so generator start and transfer stay invisible to the load: a ~10 s transfer (NFPA 110 Type 10) is nothing to a server and fatal to many PLCs. Everything upstream is someone else’s design; everything downstream is what the UPS actually protects.

Main building blocks: rectifier/charger, inverter, static bypass (millisecond path), manual maintenance bypass, battery string with DC protection, output isolation transformer where galvanic separation is required, and a monitoring card (dry contacts / SNMP / Modbus). Without a maintenance bypass, the first battery change becomes a full outage.

The three main topologies

Offline / standby

Simplest and usually cheapest. Under normal conditions the load takes utility power through the UPS; the charger keeps the battery; the inverter stands ready. On interruption (or out-of-range input), an internal switch transfers the load to the inverter.

Typical traits: low cost, compact, high efficiency on utility, limited conditioning while on mains, brief transfer to battery (often a few milliseconds — model-specific).

Fit: workstations, basic office gear, non-critical edge devices, selected DVRs/small security devices when waveform, rating and transfer suit the load.

Line-interactive

Adds voltage regulation, typically AVR via transformer taps. Within the AVR window the unit can correct moderate under/overvoltage without cycling the battery. Outside that window or on blackout, it transfers to battery.

Typical traits: better regulation than basic standby, less battery wear on shaky grids, cost/protection middle ground, still a brief transfer interval.

Fit: switches/routers, branch server racks, access-control + IT, SMB offices, selected telecom/CCTV.

Online double-conversion

Rectifies AC to DC, then inverts DC to AC; the inverter normally feeds the load continuously. On utility failure the battery feeds the DC bus and the inverter continues — no normal utility-to-inverter transfer gap.

Typical traits: continuous inverter output in double-conversion mode; strong V/f regulation within specs; suited to sensitive/critical loads; monitoring, external batteries, bypass and redundancy common; higher CAPEX and conversion losses (heat).

Fit: enterprise servers/storage, data centres, telecom, industrial automation/PLC/SCADA where compatible, critical CCTV/security, medical equipment when the specific UPS is approved for the application.

Comparison and specs that matter

Transfer times, efficiency, waveform and modes vary by product — verify the datasheet.

#	Aspect	Standby / offline	Line-interactive	Online double-conversion
1	IEC 62040-3 (typical)	VFD-SY-33	VI-SS-31	VFI-SS-11
2	Normal load path	Utility direct	Utility with AVR	Continuous inverter
3	Transfer to battery	Brief (often ~4–20 ms)	Brief (often ~2–6 ms)	No normal interruption
4	Voltage correction without battery	Limited	Yes, within AVR range	Yes, via conversion
5	Frequency regulation	Limited	Limited	Strong within specs
6	Efficiency (full load, illustrative)	>99%	~95–98%	~92–96% (eco often higher)
7	Typical cost / use	Low / home & basic office	Moderate / networking & branch IT	Higher / critical IT, telecom, industry

Also check THD (class SS caps output THD at 8% on the reference non-linear load), crest factor (~3:1 for many SMPS loads) and overload (commonly 125% for 10 min / 150% for 1 min — never assume).

Capacity: VA, kVA, watts, kW and power factor

Real power (W / kW) does useful work. Apparent power (VA / kVA) is the V×I demand. The UPS must stay inside both limits.

Power factor = kW ÷ kVA, so kW = kVA × PF and kVA = kW ÷ PF .

Example: 10 kVA at 0.8 output PF → 8 kW; at 1.0 PF → 10 kW (if other limits allow). For an 8 kW server load, a 10 kVA / 0.8 PF unit has no real-power headroom; a 10 kVA / 1.0 PF unit leaves ~2 kW nominal — still check VA limit, overload, temperature derating and load character.

Presales rule: compare actual kW and kVA output ratings, not the kVA sticker alone.

How to size capacity

Undersize → overload, bypass or shutdown. Oversize → CAPEX waste and poor efficiency at tiny loads.

1. Audit every device that must stay up — use measured watts or reliable OEM data; nameplate ≠ typical draw, but peaks/starts still count. Example: 2×2,000 W servers + 2×500 W switches = 5 kW.
2. Margin — often 20–30% by project; at 25%: 5 × 1.25 = 6.25 kW.
3. kVA — if load PF ≈ 0.9: 5 ÷ 0.9 = 5.56 kVA; ×1.25 ≈ 6.94 kVA. Need ≥6.25 kW and ~6.94 kVA.
4. Standard model — a 6 kVA / 1.0 PF unit (6 kW) still undersizes 6.25 kW; an 8 kVA / 1.0 PF unit can cover the preliminary numbers if derating and load type allow.
5. Starts / overload — motors, compressors, some printers: check overload curve, crest factor, transient response and permitted load types. Margin does not replace this.
6. Runtime separately — adequate kW/kVA does not guarantee autonomy; battery sizing is a second exercise.

Single-phase: VA = V × I → 230 × 10 = 2,300 VA = 2.3 kVA. Three-phase balanced: VA = √3 × V_L × I_L → 1.732 × 400 × 20 ≈ 13.86 kVA.

Check supply voltage/phase, load phase character, kW/kVA, phase balance, breakers/cabling/earthing, redundancy and expansion before locking 1φ vs 3φ.

Battery runtime and autonomy

Autonomy is how long the UPS supports the load after normal supply is gone. It depends on load watts, battery V/Ah and chemistry/condition, inverter efficiency, discharge rate and usable energy, temperature/age, cut-off voltage and approved external modules.

A larger UPS does not automatically mean longer runtime — a high-kVA unit may have a small internal bank; a smaller unit may carry large external cabinets.

Nominal energy: Wh = V × Ah → 48 V × 100 Ah = 4,800 Wh. Not all reaches the AC load.

Preliminary estimate: Runtime (h) ≈ usable Wh × η_inv ÷ P_load . Illustration: 4,800 Wh × 0.70 usable × 0.90 η ÷ 1,000 W ≈ 3 h — theoretical only. High-rate discharge is non-linear (Peukert); size from the manufacturer’s constant-power / runtime curves for the exact model and load.

First-pass engineering form used on many bids: t = (Vdc × Ah × η × DoD × k_age) ÷ P_load . Example: 5 kW on 192 V / 100 Ah, η 0.92, DoD 0.8, age 0.8 → 11,305 Wh ≈ 2.25 h. Add growth margin before ordering — then still verify OEM tables (W/cell at 10/15/30 min), not Ah alone.

Runtime goals: short ride-through / controlled shutdown; generator bridging; extended autonomy where no generator applies. Define duration in design — do not assume it from kVA.

Extending runtime: approved external battery modules/cabinets; shed non-essential load; UPS designed for long autonomy; UPS + generator for long outages; chemistry suited to duty. Confirm charger capacity, string layout, protection, cable size and max supported configuration before adding batteries.

VRLA vs lithium-ion

Battery chemistry drives CAPEX, footprint, life, maintenance and TCO.

#	Factor	VRLA lead-acid	Lithium-ion
1	Initial cost	Often lower	Often higher
2	Weight / footprint	Heavier, larger	Often lighter/more compact
3	Service life	Strongly temperature-dependent; often shorter in practice	Often longer in suitable duty (product-specific)
4	Recharge	Depends on charger/battery	Can support faster recharge where designed
5	Standards (typical)	IEC 60896-21/22	IEC 62619; BMS essential
6	Selection basis	CAPEX & established service	TCO, space, life

Many VRLA designs prefer ~20–25 °C ambient — life often halves about every 10 °C above 25 °C. LiFePO4-class UPS batteries may quote ~8–12 years with a sound BMS and higher cycle life, but figures are not universal — use the selected manufacturer’s data. Lithium is not “always twice the cost” or “always 10 years.”

TCO includes UPS + batteries, replacement frequency, efficiency/energy, cooling, service, floor space, downtime risk and warranty/EOL — not only purchase price.

Installation: floor loading, cooling, bypass

Floor loading. Large UPS + battery cabinets can be very heavy. Verify total weight, floor and raised-floor limits, delivery path, lifts, clearances, anchoring and any load-distribution frames from OEM drawings — get structural approval when required.

Heat and HVAC. Double conversion rejects conversion losses as heat; batteries have temperature limits. Use OEM heat rejection (BTU/h or W; 1 W ≈ 3.412 BTU/h). Cool for UPS + batteries + other room loads; poor cooling shortens battery life and reliability.

Maintenance bypass isolates the UPS for service while the load takes an alternate path. Address ratings, interlocking, isolation sequence, earthing/protection coordination, and the fact that bypass may expose the load to raw utility. Static bypass ≠ maintenance bypass: static is an internal/supported alternate path under operating conditions; maintenance bypass is for servicing. Follow OEM sequence — some transfers are make-before-break, some are not.

Critical features

Network management / shutdown software — remote alarms, logs, battery status; orderly server shutdown on long outages. Redundancy / N+1 — extra module beyond load need; two UPS units are not fully redundant if they share one vulnerable upstream or downstream path. EPO (Emergency Power Off) — may shut down inverter, disconnect output or trigger defined actions. Do not assume EPO disconnects every battery or removes all hazardous energy. Location, labelling and interlocking follow facility fire strategy, codes and OEM instructions — designed and commissioned by qualified staff.

Applications

Servers / data centres — measure real consumption, autonomy, monitoring, redundancy, access; online common where continuity and regulation are critical. CCTV / security — include PoE switches, cameras, NVR/VMS, access panels and workstations; distributed UPS for remote cabinets vs central UPS for the server room by layout and runtime. Networking / telecom — ride-through, shutdown and/or extended backup; rack space and remote monitoring. Industrial automation — PLC/HMI/SCADA may need UPS for control continuity or safe process stop; large motors/high inrush are not automatic IT-UPS loads.

UPS vs stabilizer vs generator

System	Primary purpose	Energy source	Typical role
UPS	Continuity + selected PQ for critical loads	Batteries (or approved storage)	Immediate backup / controlled shutdown
Voltage stabilizer	Correct voltage within operating range	Normally none	Regulation only
Standby generator	Long outages	Fuel / engine	Extended backup after start

A stabilizer normally has no battery. A generator needs start/stabilize time. A UPS bridges that interval. Verify generator capacity, UPS input needs, V/f behaviour, harmonics and OEM guidance — ask for input THDi and rectifier walk-in; unfiltered thyristor rectifiers can stress gensets. The “generator 1.5–3× UPS” thumb rule is too wide to price on — get written confirmation for that rectifier type.

Check that it works

• Visual, wiring/protection, Vin/Vout, phase sequence/balance verified.
• Battery autonomy checked against the design load (or OEM runtime table for that model).
• Bypass path tested under approved procedure; alarms and EPO behaviour documented.
• Monitoring card reports status; orderly shutdown software tested where specified.
• Measured final load recorded after install — not only the design estimate.

Common problems and how to fix them

Selected by kVA alone

Ignore kW/PF and the unit trips or goes to bypass under real load. Compare both output ratings and derating.

Capacity confused with runtime

Adequate kW/kVA does not guarantee autonomy. Size the battery bank separately from OEM curves.

No headroom for growth or starts

Laser printers, motors and compressors stress crest factor and overload. Keep high-inrush loads off a critical IT UPS unless assessed.

All topologies assumed equal

Standby does not condition like online double-conversion. Match IEC class and datasheet behaviour to the load.

Battery life treated as a guarantee

Generic years are not warranties. Control ambient, replace by condition, and do not mix strings of different age/capacity.

Cooling and floor loading neglected

Heat shortens batteries; heavy cabinets need structural approval. Size HVAC to UPS + batteries + room loads.

No maintenance bypass

The first battery change becomes a full outage. Design interlocking and isolation before handover.

EPO assumed to clear all energy

EPO is a defined emergency action — not always full battery disconnect. Follow facility and OEM rules.

No monitoring

Dead batteries discovered on the first outage. Fit SNMP/Modbus/dry contacts and test them.

Neutral and transfer mistakes

Neutral sized like a phase; 3-pole switching on a separately derived neutral. Confirm earthing and pole count with the UPS topology.

Commissioned only on a resistive bank

Unrealistic load hides soft failures. Verify final measured load after install.

Frequently asked questions

What does UPS mean?

Uninterruptible Power Supply — backup plus selected disturbance protection by design.

What is best for servers?

Online double-conversion is often preferred for critical servers/storage; final choice follows availability, load, budget and datasheet.

kW vs kVA?

Real vs apparent power; both UPS limits must cover the load.

How do I size for a 5 kW load?

At PF 0.9 ≈ 5.56 kVA; with 25% margin ≈ 6.25 kW and ~6.94 kVA.

Does a larger UPS mean more runtime?

No — the battery bank and load decide autonomy.

Can I add external batteries?

Only if the UPS is designed/approved for that configuration and charger.

Is lithium always better?

No — TCO, space, environment and product data decide.

Can a laser printer sit on the IT UPS?

Often high fuser inrush — keep it off the critical IT UPS unless assessed and rated.

Static bypass vs maintenance bypass?

Operating alternate path vs service isolation path.

What does EPO do?

Defined emergency action — not always full battery disconnect.

Can a UPS work with a generator?

Yes for bridging — verify compatibility (THDi, walk-in, V/f) before commissioning.

Talk to PSS about UPS systems

PSS designs, installs, and maintains UPS and battery systems — offline, line-interactive and online double-conversion — sized from measured load and runtime. Contact the PSS team to book a first site survey.