Table of Contents
- What an Uninterruptible Power Supply Does
- How a UPS Works: Standby, Line-Interactive and Online Designs
- Online UPS vs Line Interactive UPS: Which Topology Fits Your Load?
- UPS Sizing Calculator: Matching Capacity to Your Equipment
- UPS Battery Backup Runtime: What to Expect and How to Extend It
- UPS Maintenance Checklist for Critical Facilities
- Choosing the Right Uninterruptible Power Supply for Your Use Case
- Frequently Asked Questions
Last Updated: October 6, 2026
What an Uninterruptible Power Supply Does
An uninterruptible power supply sits between mains power and your equipment, supplying stored battery energy the instant the grid fails.
A UPS is not a generator: a generator takes seconds to start and needs fuel, ventilation and a changeover switch, while a UPS responds in milliseconds and bridges the gap, so the two are usually deployed together.
At Treske Pty Limited, we design, supply and install power, cooling and rack infrastructure for data centres and critical facilities.
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How a UPS Works: Standby, Line-Interactive and Online Designs
A UPS converts incoming AC power to DC to charge a battery, then converts DC back to AC to feed the load. How much conditioning happens between those steps separates the three main topologies.
Standby UPS
The cheapest design. Mains power passes straight through to the load while the battery charges; when voltage drops below a threshold, the inverter and transfer switch move the load to battery. Transfer time runs a few milliseconds, fine for a desktop but risky for equipment with sensitive switch-mode power supplies.
Line-Interactive UPS
Adds an automatic voltage regulator, usually a buck-boost transformer, that corrects brownouts and overvoltages without draining the battery. Transfer to battery only happens on a full outage or a swing the AVR cannot handle. The sensible default for workstations, point-of-sale terminals and small comms rooms.
Online Double Conversion UPS
The load never touches raw mains. AC is rectified to DC, the DC bus feeds both battery and inverter, and the inverter rebuilds clean AC continuously. There is no transfer time because there is nothing to transfer. The topology for servers, storage, pathology analysers and anything where a 4-millisecond gap matters.
Online UPS vs Line Interactive UPS: Which Topology Fits Your Load?
The choice between online double conversion and line-interactive comes down to four measurable differences: transfer time, output voltage regulation, efficiency, and how each handles power-quality problems that are not full outages.
Transfer time and what it actually threatens
A line-interactive unit transfers to battery in roughly 2 to 6 milliseconds, fast enough for almost all switch-mode power supplies, which hold their DC bus up for around 10 to 20 milliseconds at typical loading. The load that gets caught out has a very small input capacitance or a tight hold-up window: some industrial controllers, certain laboratory instruments, and high-density blade chassis.
Voltage regulation under sustained deviation
Line-interactive designs use a buck-boost transformer to correct brownouts and overvoltages in steps, bringing the output back within a workable band without draining the battery. That handles the common sag to around 200 to 210 volts, but is less effective when the supply is chronically high or low, because the AVR corrects continuously and the transformer runs warmer.
Efficiency and heat
Line-interactive units are more efficient in normal operation, commonly in the low-to-mid 90s per cent, because mains power passes through with only transformer correction. Online double conversion loses energy in two conversions and sits lower, though ECO modes can lift efficiency by bypassing the double conversion when the input is clean. The trade-off is that ECO mode reintroduces a transfer event, so enable it only where the load tolerates it.
Power-quality problems beyond outages
A line-interactive UPS addresses outages and voltage deviation, but not waveform distortion, frequency variation or high-frequency noise. An online unit rebuilds the sine wave from DC, isolating the load from harmonics, frequency drift and transients that pass through a line-interactive design. If your site shares a supply with welders, large variable-speed drives or lift motors, that distinction is the reason to specify online.
The decision, stated plainly
| Condition | Line-interactive | Online double conversion |
|---|---|---|
| Load tolerates a few milliseconds of transfer | Suitable | Suitable, but not required |
| Supply has frequent sags or swells | Suitable within AVR range | Suitable |
| Supply has harmonics, frequency drift or noise | Not addressed | Addressed |
| Load has a narrow hold-up window | Risky | Correct choice |
| Efficiency and heat are the priority | Better | Lower, unless ECO mode is used |
A common mistake is buying line-interactive for a server rack because the runtime figures look similar on paper. Runtime is not the differentiator; transfer time, output voltage regulation and harmonic handling are. Moving from standby to online also means a different acoustic profile and a higher heat load in the rack, so plan the airflow before the delivery date.
UPS Sizing Calculator: Matching Capacity to Your Equipment
Sizing a UPS means adding up the real power draw of everything you intend to protect, converting to volt-amperes using the load's power factor, then adding headroom. Under-sizing causes nuisance overload shutdowns. Over-sizing wastes capital and runs the battery inefficiently.
Work through it in this order:
- List every device and its rated wattage from the nameplate.
- Apply a realistic load factor. Nameplate ratings are maximums, not typical draw.
- Sum the watts, then divide by the power factor to get VA. A power factor of 0.9 is a reasonable assumption for modern IT equipment; check the actual figure for anything unusual.
- Add 25 to 30 per cent headroom for growth and inrush.
- Confirm the UPS output power factor matches or exceeds your calculated load.
Step-by-Step Sizing Example
Say you are protecting a small server room: one 500 W server, one 150 W network switch, one 200 W storage array and a 100 W monitor. Total: 950 W. Divide by 0.9 to get approximately 1,056 VA; add 30 per cent headroom and you land near 1,370 VA, so a 1,500 VA or 2 kVA unit is the right bracket.
UPS Battery Backup Runtime: What to Expect and How to Extend It
Runtime is battery capacity divided by load, which means it falls faster than most people expect as load rises.
Most small UPS units deliver five to fifteen minutes at full load, not to run your business through an outage, but to ride through a short interruption or allow a graceful shutdown.
To extend runtime you have three options: add external battery modules, reduce the connected load to essentials only, or move to a larger frame with more battery capacity.
Eaton 9PX Series UPS 1kVa - 3kVA Online Double Conversion →
Battery Lifespan, Replacement and Total Cost of Ownership
Valve-regulated lead-acid batteries typically last three to five years in a temperature-controlled room, and considerably less in a hot rack.
Lithium-based units cost more upfront and generally last longer, with better tolerance for higher ambient temperatures. Run the numbers over a ten-year horizon rather than comparing sticker prices. Replacement batteries, freight, disposal and technician time all belong in the calculation.
UPS Maintenance Checklist for Critical Facilities
A maintenance schedule turns a UPS from a box in the corner into a dependable asset. The work is unglamorous and it is the single highest-return activity in any power protection program, but most checklists leave out the installation and placement context that determines whether the maintenance is even effective.
Safe installation and placement
Placement decides half the outcome before the unit is ever energised.
- Keep the unit out of the hottest part of the rack. Heat rises, so the top of a cabinet is usually the worst position for battery life.
- Leave the manufacturer's specified clearance at the front and rear for airflow. Never block the front intake with cable bundles or blanking panels.
- Confirm the circuit can carry the UPS plus its recharge current, not just the connected load. A UPS recharging a depleted battery draws more than its steady-state input.
Battery handling and replacement precautions
Valve-regulated lead-acid batteries are safe in normal use but unforgiving of careless handling. Before any battery work, isolate the string, confirm zero voltage at the terminals, and remove metal jewellery. Do not short the terminals with tools. Swollen or hot cells should be removed from service immediately and disposed of through a licensed recycler, not stored in the rack room.
Monitoring, software and graceful shutdown
Monitoring is where most sites leave value on the table. Modern units expose status over SNMP or a network card, feeding directly into your existing monitoring platform. The practical setup:
- Connect the UPS to the network via a management card, not just a USB cable to one server.
- Configure alerts for battery low, on-battery, overload, overtemperature and self-test failure, and route them to a monitored inbox rather than an individual.
- Install the vendor's shutdown agent on every protected host and set the threshold above the point where the battery can no longer carry the load.
The maintenance checklist
- Record battery string voltage and compare against baseline
- Run a timed discharge test at a representative load
- Check terminal torque and inspect for corrosion or swelling
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For sites without in-house capability, a structured preventative maintenance program covers battery testing, load banking and reporting. Treske Pty Limited provides preventative maintenance for critical power infrastructure, along with UPS installation and commissioning for new deployments.
Choosing the Right Uninterruptible Power Supply for Your Use Case
Match the topology and capacity to the load, then match the unit to the environment.
For a small server room or comms cabinet, step up to online double conversion. The Eaton 9PX Series from 1 kVA to 3 kVA is a rack or tower convertible unit with a graphical LCD, scalable runtime through additional battery modules, and management software for remote monitoring and graceful shutdown.
For medical, laboratory and industrial environments where the supply is unstable and the load is unforgiving, the higher end of the Smart-UPS SRT range or a comparable online unit with matching battery packs is the right answer.
| Use Case | Recommended Topology | Typical Capacity |
|---|---|---|
| Workstation, POS terminal | Line-interactive | 550 VA - 1600 VA |
| Small server room | Online double conversion | 1 kVA - 3 kVA |
| Rack with blade servers | Online double conversion | 5 kVA - 20 kVA |
| Medical or lab equipment | Online double conversion | Sized to load |
Power failures do not schedule themselves, and the cost of an unplanned shutdown in a critical facility runs well beyond the hardware.
Frequently Asked Questions
How long can a UPS last without power?
Runtime depends on load and battery capacity. A small line-interactive unit might give 5-15 minutes for a workstation, while an online double conversion system with extended battery modules can run critical servers for hours. For example, the Eaton 9PX Series supports scalable runtime by adding battery modules. To estimate, divide battery capacity by your load in watts, then factor in efficiency losses of roughly 10-15%. Always test under real load to confirm actual performance.
What is the difference between standby, line-interactive and online UPS systems?
Standby UPS switches to battery only during an outage, offering basic surge protection. Line-interactive adds automatic voltage regulation to correct brownouts and overvoltages without draining the battery, making it suitable for workstations and POS equipment. Online double conversion constantly converts AC to DC and back, providing the cleanest power and zero transfer time. It suits servers, medical labs and any load that cannot tolerate even a brief switchover. The right choice depends on how sensitive your equipment is to voltage fluctuations and transfer time.
How do you calculate the right UPS size for your equipment?
Add up the wattage of every device you plan to connect, then divide by the UPS power factor to get volt-amperes (VA). For example, a 600W load with a 0.9 power factor needs at least 667VA. Add 20-30% headroom for future growth and to avoid running the UPS at full capacity. A UPS sizing calculator can simplify this, but always verify against the manufacturer's specifications. For loads above 3kVA, an online double conversion model like the Eaton 9PX Series is a reliable starting point.
What should not be plugged into a UPS?
Avoid connecting laser printers, space heaters, vacuum cleaners, large motors or any device with a high inrush current. These can overload the UPS and drain the battery quickly. Also skip surge protectors and power boards plugged into the UPS output, as they can interfere with the UPS's own protection circuitry. Focus on critical equipment: servers, routers, switches, security cameras, monitors and storage devices. If you need to back up a laser printer, use a dedicated circuit instead.
How often should UPS batteries be tested or replaced?
Test batteries every six months with a runtime calibration or load bank test. Most sealed lead-acid batteries last three to five years, while lithium-ion options can reach eight to ten years. Replace batteries when runtime drops below 80% of the original specification or when the UPS reports a battery fault. A preventative maintenance schedule that includes battery testing, torque checks and firmware updates will catch degradation early and prevent unexpected failures during an outage.
Can a UPS be used with a generator?
Yes, but the generator must produce stable, clean power. Online double conversion UPS models handle generator power best because they convert incoming AC to DC and back, isolating your equipment from frequency and voltage swings. Line-interactive and standby designs may switch to battery repeatedly if generator output is unstable, draining the battery. Size the generator to at least 1.5 times the UPS rating to account for startup surges. Always test the combined system under load before relying on it.
What maintenance tasks should be on a UPS checklist?
A thorough UPS maintenance checklist covers visual inspections for dust or corrosion, battery voltage and impedance testing, runtime calibration, fan and filter cleaning, torque checks on terminals, firmware updates, and verifying monitoring software alerts. For online double conversion units, also check transfer time and bypass operation. Schedule this quarterly for critical facilities and annually for smaller installations. Keep a log of every test result to spot trends before they become failures.