What "critical power" means
Critical power is the part of a facility's electrical supply that a UPS, its batteries and the surrounding distribution architecture exist to protect from any interruption the connected load cannot tolerate, not just from a total blackout.
Load classification
Not every load behind a switchboard needs the same protection. A useful, widely used way to group loads by how badly an interruption hurts is:
Critical
Zero tolerance for any break, sag or waveform disturbance. Loss causes immediate data loss, process failure or safety risk. Always fed from an online (double-conversion) UPS with battery backup. Example: core compute/storage, process control systems.
Essential
Must keep running to support the critical load or the space around it, but can usually tolerate a very short, sub-second break. Example: UPS room cooling, some control and monitoring systems.
Non-essential
Desirable to keep running but can be shed for a period during a contingency without an immediate operational failure. Example: general office IT, non-critical displays.
Non-critical
Ordinary building load with no UPS backing at all; it may still sit on the generator, but never on the UPS. Example: general lighting, general small power.
Getting this classification right up front drives almost every later decision in this tool: which UPS topology to specify, how much redundancy to design in, and where the boundary between "battery-backed" and "generator-backed only" actually sits on the single-line diagram.
Redundancy concepts: N, N+1, 2N, 2N+1
Redundancy describes how much spare capacity or how many spare paths exist beyond what is strictly needed to carry the load, so that a single failure or a piece of planned maintenance does not interrupt the critical load.
| Configuration | What it means | Survives one module failure? | Survives planned maintenance without a break? | Relative cost |
|---|---|---|---|---|
| N | Exactly the capacity needed to carry the load, no more. All installed capacity is required simultaneously. | No | No | Lowest |
| N+1 | One extra module of the same size beyond what N requires, sharing a common bus. | Yes, one module | Yes, one module can be pulled for service | Moderate |
| 2N | Two entirely independent, fully rated systems (or buses), each able to carry 100% of the load alone. | Yes, an entire system | Yes, one whole system can be shut down | High (double capacity) |
| 2N+1 | Two independent systems as in 2N, but each of those systems is itself built with N+1 redundancy internally. | Yes, an entire system plus a module in the surviving system | Yes, with margin to also service a module in the remaining system | Highest |
Simple way to picture it: N is "just enough", N+1 adds one spare part to a shared pool, 2N duplicates the whole pool, and 2N+1 duplicates the whole pool and still adds a spare part to each copy. Tab 08 walks through N+1 and 2N side by side with an interactive failure simulation.
Single points of failure (SPOF)
A single point of failure is any one component whose failure alone takes the critical load down, regardless of how much redundancy exists elsewhere in the chain. Designing out SPOFs means tracing the full path from utility to the load's power cord and checking every link:
- Single utility feed - one substation or one incoming feeder with no second independent source or generator backup.
- Single UPS module with no redundancy - an N-only UPS system where any internal fault drops the load straight to bypass or to nothing.
- Single battery string - one string feeding the whole DC bus, so a single failed string (or a maintenance disconnect) removes all battery backup.
- Single static transfer switch or bypass path - if every module ultimately depends on one shared switch or one shared distribution board, that shared component is the new SPOF even after the UPS modules themselves are made redundant.
- Single-corded load - even a perfect 2N power system fails the load if the equipment itself only has one power supply cord plugged into only one of the two independent paths.
Verify before design: the required redundancy level, the acceptable load classification for a given space, and where the critical/non-critical boundary sits are project-specific decisions driven by the client's risk tolerance, applicable codes and (where targeted) data center tier or similar certification requirements. Confirm against the current project specification, not against this training material.
The three IEC 62040-3 UPS topologies
IEC 62040-3 groups UPS behavior into three broad classes based on how independent the output is from the incoming mains. Click a topology below to see its block diagram and how it behaves.
Comparison
| Topology | IEC 62040-3 class | Transfer time | Typical efficiency | Protection level | Typical use |
|---|---|---|---|---|---|
| Offline / Standby | VFD (voltage and frequency dependent) | ~2-10 ms | Very high, ~97-99% | Blackout and deep sag/surge only; frequency variation passes through | Desktop PCs, small POS terminals, non-critical single workstations |
| Line-Interactive | VI (voltage independent) | No break for normal sag/surge (AVR correction); ~2-4 ms if battery transfer is needed | High, ~95-98% | Corrects voltage sag/surge via tap-changing without going to battery; frequency still passes through | Small-medium server rooms, network closets, SMB/branch office IT |
| Double-Conversion Online | VFI (voltage and frequency independent) | 0 ms - load is continuously fed by the inverter | Historically lower, ~90-96% (higher in eco/hybrid modes) | Full isolation from all incoming voltage, frequency and waveform anomalies | Data centers, hospitals, critical process/manufacturing, trading floors |
Explore each topologySelect one to see how power actually flows through it
In normal operation the load is fed almost directly from filtered mains through a simple switch. The inverter and battery sit idle, only connecting when mains fails or sags/surges beyond a wide tolerance band, which is why there is a short, few-millisecond transfer time. Because the inverter is off most of the time, efficiency is very high, but frequency variations and moderate voltage disturbances pass straight through to the load unfiltered.
The inverter stays electrically connected in the power path all the time, working alongside a tap-changing autotransformer (AVR) that automatically corrects moderate voltage sag or surge without ever calling on the battery. The battery and inverter only fully take over the load if mains drops outside the AVR's correction range or fails outright, giving a very fast (or effectively no-break) transfer for the disturbances it is designed to handle.
Mains power is continuously rectified onto a DC bus, which floats the battery and continuously feeds an inverter that regenerates a fresh AC waveform for the load, all the time, in both normal and outage conditions. There is no "transfer event" in the normal sense, since the inverter never stops supplying the load, which is why transfer time is quoted as effectively zero. A static bypass switch stands by to feed the load directly from raw mains only if the UPS itself faults or is overloaded.
UPS sizing calculator
Build a load list, add a growth/spare margin, and choose a redundancy configuration. The calculator converts kW to kVA, applies the margin, and shows how the required capacity splits across modules under N, N+1 or 2N.
Load list
Enter each item's value in either kW or kVA (choose the unit per row). For rows entered in kW, provide the load's power factor; if left blank, 0.9 is assumed (typical for modern IT/server loads - use the nameplate or measured value where known). Quantity multiplies the row.
| Item name | Value | Unit | PF | Qty | Row kVA |
|---|
Margin and redundancy
Sizing formulas
Enter a load list to see a sizing note here.
Verify before selection: this is a training-level approximation. Final UPS selection must account for inrush/inductive load characteristics, the manufacturer's actual module ratings and de-rating at altitude/temperature, input harmonic performance against generator or utility limits, battery sizing for the required runtime, and the project's real load schedule and diversity, not this simplified tool.
Battery runtime calculator
A simplified, linear estimate of backup runtime from battery bank capacity, system voltage, UPS efficiency and a derating factor for aging/temperature.
System voltage
Set the DC bus voltage either by entering the number of 12V battery blocks connected in series, or by entering the total system voltage directly.
Common UPS DC bus voltages are built from strings of 12V blocks: 20 blocks ≈ 240V, 32 blocks ≈ 384V, 40 blocks ≈ 480V are typical examples, though exact values vary by manufacturer and float voltage per block.
Load and battery condition
This is a simplified linear estimate only. Real battery runtime does not scale linearly with load; it depends on the battery's published discharge curve at the actual C-rate being drawn (capacity in Ah falls as discharge current rises), end-of-discharge voltage, temperature, and true age/health of the string. Always verify actual expected runtime against the manufacturer's discharge chart for the specific battery model at the specific load, not this tool.
Battery technology comparison
Three battery families cover the large majority of stationary UPS applications, each with a different balance of lifespan, footprint, maintenance and cost.
| Type | Typical design life | Footprint / weight | Maintenance | Temperature sensitivity | Relative cost | Typical application |
|---|---|---|---|---|---|---|
| VRLA (AGM / Gel) | ~3-5 years standard, up to ~8-10 years on "long life" product lines | Moderate; sealed, stackable in cabinets or open racks | Sealed/valve-regulated, no watering; still needs periodic visual inspection, internal impedance testing and a capacity (discharge) test | High - life roughly halves for every ~8-10C sustained above 25C (a commonly cited rule of thumb) | Lowest upfront cost | Most commercial UPS installations; small to mid-size data centers and comms rooms |
| Vented Lead-Acid (VLA / flooded) | ~15-20 years typical | Larger and heavier per kWh than VRLA; needs a dedicated, ventilated battery room | Requires periodic watering/electrolyte checks, terminal cleaning and specific gravity checks; higher labor | Prefers ~20-25C but tolerates excursions somewhat better than VRLA relative to its long life | Higher upfront and ongoing O&M cost, lowest cost per year over its long life | Large data centers, telecom central offices, utility substations, long-design-life critical facilities |
| Lithium-ion (LFP) | ~10-15 years typical | Much smaller footprint, roughly 1/3 to 1/2 the weight of VRLA for equivalent energy | Minimal; BMS-managed, no watering, but depends on the battery management system and its thermal/balancing functions operating correctly | Wider safe operating range and less capacity loss with heat than VRLA, still benefits from stable, moderate temperatures | Highest upfront cost; often lower total cost of ownership from lifespan and space savings | Modern/hyperscale data centers, colocation, retrofit projects with floor-loading or space constraints |
These are general, widely cited characteristics for training purposes only. Always confirm design life, temperature limits and maintenance requirements against the specific manufacturer's datasheet for the model actually selected.
Static transfer switch (STS) & bypass
Three related but distinct mechanisms keep a critical load fed even when the UPS itself needs to be taken out of the power path, briefly or for extended maintenance.
Automatic bypass, maintenance bypass, and STS
Automatic (static) bypass
Built into the UPS itself. If the inverter faults or is overloaded beyond its rating, an internal static switch automatically transfers the load to raw mains within milliseconds, with no operator action. It exists purely to protect the load from an internal UPS problem, not to allow physical maintenance.
Maintenance bypass
A manually operated external (or internal) switch/breaker set that fully isolates the UPS, both its input and output, while the load continues to be fed directly from raw mains. This is what allows a technician to safely work on the UPS itself with it completely de-energized from the load path.
Static transfer switch (STS)
A separate downstream device (not part of a single UPS) that switches a dual-corded load between two independent upstream sources, typically two separate UPS systems or buses, in a few milliseconds if the preferred source fails or drifts out of tolerance. It protects against loss of an entire upstream source, not just an internal UPS fault.
Interactive power flow diagramSelect a state to see the active current path highlighted
Data center tier classification concepts
A widely referenced way to describe how much redundancy and concurrent maintainability a data center's infrastructure, including its critical power chain, is designed to provide.
| Tier | Redundancy level (general concept) | Concurrently maintainable | Fault tolerant | Illustrative annual downtime (order of magnitude) |
|---|---|---|---|---|
| Tier I | N - basic capacity, single distribution path | No | No | ≈28.8 hours/yr |
| Tier II | N+1 components, but still a single distribution path | No | No | ≈22.0 hours/yr |
| Tier III | N+1, multiple distribution paths but only one active at a time | Yes | No | ≈1.6 hours/yr |
| Tier IV | 2N or 2N+1, multiple simultaneously active independent distribution paths | Yes | Yes | ≈0.4 hours/yr |
General concept only, not a certification standard. These rows describe widely taught, general industry concepts associated with data center tiering, not the proprietary certification criteria of the Uptime Institute or any other body. Formal Tier certification requires that organization's own defined process, documentation and site audit. Never present training-tool figures as evidence of, or a substitute for, an actual Tier certification.
Parallel & redundant UPS configurations
N+1 parallel redundancy and 2N (dual bus) architecture both protect against a single failure, but they do it very differently. Toggle between them and simulate a failure to see how each responds.
Three or more UPS modules of equal rating connect to a common output bus, each normally carrying an equal share of the load. The "+1" module carries load too, in normal operation, rather than sitting idle, so all modules share the burden evenly. If a module fails, the remaining modules automatically pick up its share as long as the system was sized so the surviving modules alone still cover the full load.
All modules healthy. Click "Simulate a failure" to see how this configuration responds.
Harmonics & power quality
UPS systems both consume and produce power in ways that are not purely sinusoidal, which matters at both the input and the output side of the system.
Input current harmonics (what the UPS draws from mains or generator)
A UPS rectifier is a non-linear load: it draws current in pulses rather than a smooth sine wave, which shows up as harmonic distortion (THD) on the input current. Older, simpler 6-pulse rectifiers can produce input current THD in the region of 25-30% if unfiltered. This matters because harmonic current causes extra heating in upstream transformers and cables, can distort the voltage waveform seen by other equipment sharing the same bus, and can upset a standby generator's voltage regulation, especially when the UPS load is a large fraction of the generator's rating.
Two common ways to reduce input harmonics are a 12-pulse rectifier arrangement (using a phase-shifting transformer to cancel certain harmonic orders) and, more commonly in modern designs, an active front end (IGBT-based) rectifier, which can bring input current THD down to roughly 3-5% and also allows the UPS to run closer to unity input power factor.
Output waveform quality (what the UPS delivers to the load)
A double-conversion (online) UPS regenerates the output waveform from its inverter, so its output THD is largely independent of how distorted the incoming mains is, and independent of how non-linear the downstream load is, up to the inverter's rated capability. Manufacturers commonly quote output voltage THD in the low single digits (often under 3% into a linear load, and typically still within single digits, commonly cited as roughly 5-8%, into a realistic non-linear IT load). Sensitive electronic equipment, particularly older switch-mode power supplies and some medical or lab instrumentation, is specified against a maximum acceptable output THD, which is one reason online UPS is preferred ahead of offline or line-interactive designs for genuinely critical loads.
Specific THD percentages and pulse-count arrangements vary by manufacturer and model. Always confirm actual input/output harmonic performance from the selected UPS's datasheet and, where relevant, a harmonic study of the upstream distribution and generator, rather than the general figures above.
Grounding & bonding for UPS systems
Correct grounding protects people and equipment; incorrect or ambiguous grounding through transfer and bypass paths can create nuisance faults, circulating currents, or a genuine safety hazard.
Single-point ground concept
A properly designed low-voltage system should have exactly one point where the neutral conductor is bonded to earth ground, normally at the source (a transformer or generator winding). Multiple neutral-ground bonds create parallel paths for neutral current to flow through ground and structural steel, which can cause nuisance ground-fault trips, circulating currents, and difficulty locating real faults.
This becomes a live design question wherever a transfer switch, STS or bypass path can connect the load to more than one source: if the switch only breaks the phase conductors and leaves the neutral solidly connected across both sources ("3-pole" switching), and both of those sources also happen to have their own neutral-ground bond, the result is two bonding points effectively in parallel. A 4-pole switch (which also switches the neutral) is one common way to guarantee only one neutral-ground bond is ever active at a time, but the correct choice depends entirely on how the specific sources, generator, UPS, and switchgear are grounded upstream, which is a system-level design decision, not a rule of thumb to apply blindly.
Battery room ventilation (hydrogen gas)
Lead-acid batteries, both vented (VLA) and to a lesser extent sealed (VRLA), release hydrogen gas during charging, especially during equalize/boost charging. Hydrogen is flammable at concentrations as low as roughly 4% by volume in air (its lower flammable limit), so battery rooms and enclosures are ventilated to keep hydrogen concentration well below that limit at all times, with margin, using calculated air change rates based on the battery bank's maximum charging current.
- Vented (flooded) batteries off-gas considerably more hydrogen than sealed VRLA under equivalent charging conditions, and are correspondingly more likely to require a dedicated, mechanically ventilated battery room.
- Sealed VRLA batteries still vent some hydrogen through their pressure relief valves, particularly under fault or overcharge conditions, so spacing, general room ventilation and the avoidance of ignition sources near the batteries remain important even for "sealed" batteries.
- Battery enclosures and rooms should avoid sources of ignition (sparks, open flame, uncontrolled hot work) and should be designed and ventilated in line with the applicable electrical and fire codes for the jurisdiction and the specific battery chemistry installed.
Verify before design: required ventilation rates, room classification, and single-point grounding arrangements for a specific transfer/bypass topology are project-specific, code-specific determinations. Confirm against the current project specification, the applicable code edition, and the battery/UPS manufacturer's installation requirements, not this training reference.
Commissioning & testing checklist
Tick items off as they are completed and verified on site. This is a training-level checklist, not a substitute for the project's approved commissioning procedure.
Assessment
16 questions covering the whole tool. Select an answer for each question, then press Check Answers.