UPS Buyers Guide Comparing Online vs Line Interactive vs Standby UPS

Which is the right UPS topology to protect your IT application? Choosing the wrong UPS can be expensive and leave a critical load unprotected when downtime is not an option. A UPS system that looks perfectly adequate and seems to perform when the mains power supply is present, can all too often deliver an incompatible waveform or output voltage, a long transfer time to battery, or a shorter than expect battery duration.

IT devices, including traditional and AI servers, IT SAN, storage solutions, bridges and routers have their own load characteristics; each needs to be matched to the right UPS topology. Deciding which UPS topology is right for your application can be distilled down into a short-set of general questions.

Why Topology Matters More Than Runtime or Capacity

There are three core UPS Topologies, and each provides a different level of power protection.

Most UPS buying guides start by asking ‘how much is my IT load in VA or Watts?’, followed by ‘how long should the battery last during a mains power outage?’. These are valid questions, but the choice of UPS topology determines the grade of electrical power provided by the UPS system and how it responds to power pollution (spikes and electrical noise), power disruption (sags, surges and brownouts) and power outages. Your IT loads can fail under duress, unable to operate properly when supplied with the wrong output waveform from your chosen UPS, or with poorly controlled and regulated voltage and frequency.

When sizing a UPS, it is always preferable to run to about 80% of the total UPS capacity. The lower the load VA or Watts, the longer the battery runtime. Often a poorly sized UPS system with the right topology can outperform an oversized UPS system of the wrong topology.

For help in sizing an uninterruptible power supply, please see our UPS Sizing Guide.

So, what are the three types of UPS technology available?

Standby UPS (Offline UPS)

A UPS solution that provides entry-level power protection. Under normal mains power conditions, the load is connected directly through a standby UPS to the mains power supply. The UPS is designed to switch to battery power (within 2-10 milliseconds) if the mains power supply fails or fluctuates outside a preset window, or if the UPS faces surge voltages, spikes, or electrical noise that its built-in filters can ‘clip’ or suppress to safer levels.

• Strengths: a simple, low-cost, energy-efficient power backup solution.
• Weaknesses: the load transfer time (mains power to inverter battery) can disrupt power to the connected load, especially sensitive IT loads that can reboot or suffer an operational data processing or storage error. The UPS provides minimal protection against sags, surges, or polluted power, and has no output waveform correction. The output is typically a step-wave or square-wave.

A “Standby of Off-line UPS”: https://www.serverroomenvironments.co.uk/standby-ups can be used to provide some protection and battery backup to desktop workstations, EPoS terminals, small IT peripherals (bridges and routers), and non-critical office equipment within a home or small office environment.

Line Interactive UPS

Offers the next level of power protection (intermediate grade) as a line interactive UPS has an automatic voltage regulator (AVR) to correct for sags, surges and brownouts, with superior filtering for spike and electrical noise protection, a quicker inverter transfer time of 2-4 milliseconds, and an improved output waveform: sometimes a sinewave.

• Strengths: this type of UPS provides superior protection to an offline UPS when mains power is present, using its AVR to smooth out voltage-related problems and reduce battery usage. The cost of operation can be lower than an online UPS as the inverter only runs when required and can be more affordable for the same capacity.
• Weaknesses: there is still a transfer time when the battery kicks in and though smaller than a standby UPS, it can disrupt some sensitive IT loads. The AVR adds a good power stabilisation effect, but it cannot handle major voltage surges or frequency variations. The output waveform in battery mode depends on the inverter section and can be square-wave or sinewave. If a longer runtime is required, the UPS is generally oversized.

Line Interactive UPS can be used in small server rooms, comms rooms and branch office server racks, where there is a relatively stable mains power supply from the local utility. Line interactive UPS up to 3kVA are among the most installed in commercial office environments, as they are plug and play and offer a good level of power protection.

Online (Double-Conversion) UPS

Whilst Off-line and Line Interactive UPS are relatively similar in design, the On-line UPS represents a fundamental technology change. Principally, the inverter continuously runs and is connected to the load. Secondly, an online UPS has a built-in automatic bypass and will fail safely to mains if there is an overload or internal UPS fault.

An online UPS is known as a double-conversion UPS as the incoming AC is converted into two levels of DC: one to charge the battery and the other to provide a DC supply to the inverter. When the mains power supply fails, capacitance within the UPS design ensures continuity of the DC supply as the inverter input changes between DC power sources. There is zero disruption to the inverter, and as the inverter is constantly running, zero transfer time for the load. It is a true ‘no-break’ supply whether running on AC mains power, generator power or the connected battery set.

• Strengths: on-line UPS provide the ultimate grade of power protection, with a guaranteed no-break supply (zero transfer time), digitally generated true sinewave inverter output and isolation from mains-borne spikes and electrical noise. The AC input operates over a wide voltage range, to reduce the need for the UPS to draw on the DC supply of its battery set. As the UPS inverter is rated for continuous running, online UPS are the preferred choice for longer runtimes.
• Weaknesses:a higher upfront and potentially higher running cost than a line interactive UPS, with online efficiencies of 95-96% achievable. The higher the electrical efficiency, the more the additional load for local cooling, and this is directly proportional to the UPS size for a given operating efficiency.

“Online UPS”: https://www.serverroomenvironments.co.uk/online-ups are the most installed within server rooms and data centres, and any critical environment that can suffer downtime. Online UPS are available for loads from 500VA to over 1MVA and come in single-phase and 3-phase UPS variants, with monobloc and modular variants.

UPS Topology Comparison Table

Note*: excludes delivery and VAT.

Another aspect to consider is the size and weight of the UPS as sometimes the space available can limit the choice of system.

How to Decide

The best way to decide on the right UPS design to protect your application is to ask three questions.

1. Can my load potentially tolerate a few milliseconds dip in power and continue to operate, or can I continue to operate my IT network if I momentarily lose a switch or router? If so, a standby or line-interactive UPS should be suitable.
2. How stable is my incoming main power supply? Not all sites are the same. Older buildings and shared industrial sites can suffer electrically from the operation of heavy machinery, rural sites suffer more brownouts and sags due to the use of more overhead electricity cables, and office buildings can be prone to power pollution from other tenants and systems.
3. What is the cost of downtime? Calculate lost revenue or service impacts from your IT not being available due to a power outage.

For server rooms and data centres, an online UPS is the best and safest option. Line interactive UPS offer a good alternative where budgets are restricted, and offline UPS should only be used for less critical IT peripherals and home-office systems.

For more information, please contact our Projects team. We supply UPS sytems from leading manufacturers including APC by Schneider, Eaton, Legrand, Riello UPS, Tecnoware and Vertiv.