UPS Types: Offline, Line-Interactive and Online Double Conversion

An uninterruptible power supply sits between the mains and the equipment you cannot afford to lose, and its whole job is summed up by a network operator's own advice: "UPS systems will provide backup power to your computer until your electricity supply is restored."¹ UK Power Networks suggests one for anyone who works from home or runs a small business from the house and uses their computer, because a cut at the wrong moment costs more than the hardware.¹

There are three main types, and the difference between them is not battery size or price but how they handle the mains. An offline, or standby, unit passes mains power straight through and only steps in when it fails. A line-interactive unit does the same but can also correct mains voltage that drifts too high or low. An online double conversion unit never passes the mains through at all: it rebuilds the output continuously from its own inverter, so there is no switchover to notice. Those three behaviours, and the milliseconds between them, are what you are actually paying for when you move up the range.

What a UPS does, in plain terms

A UPS is a short-duration bridge. It holds a battery, and when the mains fails it feeds your equipment from that battery long enough for you to save work, shut down cleanly, or wait out a brief interruption. It is not a generator and it is not a home battery: its purpose is seconds to minutes of clean, continuous power for a small, chosen set of loads, not hours of whole-house supply. UK Power Networks frames it exactly that way, as backup "until your electricity supply is restored", and pairs the advice with a note that surge protectors can shield equipment such as landline phones and internet routers from voltage variations.¹

That pairing matters, because a power cut is only one of the things the mains does to a home. Voltage can sag, spike and wander, and the three UPS types respond to those events differently. The same network operator page that recommends a UPS for home workers also points households to compensation routes when a power cut damages something in the home, which is the other half of the story: a UPS reduces the chance of damage in the first place.¹ For the wider picture of what a cut does to a house, see our pillar page on backup power for UK homes, and for choosing a unit for a specific room, home UPS systems.

Standby UPS: the most basic design

The standby UPS meaning is in the name. In normal operation the unit is essentially a pass-through: mains power flows to your equipment, the battery is kept charged, and the inverter sits idle. Only when the mains fails or falls outside acceptable limits does the inverter start and the load transfer to the battery. That transfer takes a few milliseconds, which most computer power supplies ride through without noticing, but it is a real gap, and it is the defining limitation of the type.

The payoff is simplicity and price. Because the inverter only runs during an outage, an offline unit runs cool and quiet the rest of the time, wastes little power and costs the least of the three types. For a single desktop PC, a monitor or a broadband router, that is often all a household needs: the equipment is tolerant of a brief gap, and the UPS exists to cover the cut, not to polish the mains. The same logic appears in the network operator's advice, which recommends a UPS for the home computer without distinguishing type, because for that load the cheapest topology does the job.¹

What an offline unit does not do is regulate. If the mains voltage sags or swells while staying within the transfer thresholds, the load sees it. Protection against voltage spikes is a separate job entirely: NICEIC notes that surge protective devices in the consumer unit "protect circuits and equipment from high-voltage power surges, such as those caused by lightning", which is a different failure mode from an outage and needs a different device.⁵ Our page on surge protection and lightning covers that layer.

Line-interactive UPS: voltage control via an AVR

A line-interactive UPS adds one thing to the standby design: automatic voltage regulation, usually written AVR. The unit still passes mains power through in normal operation, but a transformer tap arrangement raises or lowers the voltage when the mains drifts, without switching to battery. The battery is reserved for actual outages, which extends its life and keeps the connected equipment running through brownouts and overvoltage that would push an offline unit onto battery or let the drift through.

The maker pages for this class show how wide the working window is. Legrand's Keor DC single-phase line-interactive UPS lists an input voltage range of 90 to 264 V, a figure that tells you the unit keeps working across a very wide spread of incoming mains conditions.⁶ The same product page lists its regulation type as portable standard format, a DC voltage type, and a load type of "Universal and LED Retrofit", which places this particular unit among the smaller, plug-in end of the range rather than a whole-office system.⁶

For a household, the line-interactive class is the middle of the road in every sense: more capable than offline where the local supply is unstable, cheaper and cooler-running than online. It is the topology most often found in units sold for home offices and small server cupboards, and the AVR is the feature worth paying for if your area suffers voltage sags rather than clean cuts.

Online double conversion UPS: no transfer time at all

An online double conversion UPS does something more radical. Incoming mains power is rectified to DC and then inverted back to AC continuously, so the load never draws from the mains directly. The battery sits on the DC link, which means that when the mains fails there is no transfer at all: the load was already running on the inverter, and it keeps running. This is what "zero transfer time" means, and it is the only topology that can honestly claim it.

The second benefit is output quality. Because the output is rebuilt from scratch, the load sees a steady, clean waveform regardless of what the mains is doing, which is why double conversion is the standard choice for servers, network rooms and sensitive equipment. The costs are continuous conversion losses, heat and noise, since the inverter runs every hour of every day, and a higher price for the electronics that make it possible.

Maker claims for switchover performance in adjacent products give a sense of where the bar sits. SAJ states a switchover time of no more than 10 ms for its AS2 series, "ensuring uninterrupted power during outages".² That is a fast transfer, not zero, and the difference between 10 ms and nothing is exactly the gap the online topology exists to close. For loads that cannot tolerate any gap at all, double conversion is the answer; for everything else, it is insurance against a problem the cheaper types have already largely solved.

Transfer time: the number that separates the three types

Transfer time is the gap between the mains failing and the UPS taking over, and it is the cleanest way to understand why the three types exist at all. Offline units have the longest gap, because the inverter must start from idle. Line-interactive units are similar for outages, though the AVR handles voltage drift without any transfer. Online units have none, because the load was never on the mains in the first place.

The figures that makers publish cluster in single-digit milliseconds. SAJ's AS2 series claims no more than 10 ms², and AlphaESS describes the SMILE-G3 home battery's sub-20 millisecond switch-over as UPS-grade backup³, which tells you the band that equipment makers treat as acceptable for computers and network gear. A desktop PC's power supply bridges a gap of that length, which is why offline and line-interactive units dominate the home market. Firmware updates show the same priority: a Fronius Symo GEN24 release in June 2024 listed improved backup switching time among its changes, evidence that makers treat switchover speed as a live performance metric even in solar inverters.⁷

What transfer time does not cover is duration. A fast transfer keeps a router alive through the moment of the cut; only battery capacity keeps it alive through the hours after. Those are separate purchases, and confusing them is the most common mistake in choosing a UPS. Our page on UPS sizing deals with the second question, and island mode and black start explains how larger backup supplies take over a whole circuit.

Matching each UPS type to household or office loads

The right topology follows the load, not the house. RdSAP, the standard used to classify UK dwellings, sorts homes into house, bungalow, flat, maisonette and park home⁸, and the same logic of matching kit to context applies here: what you are protecting determines what you need.

• Broadband router, modem, VoIP phone. Very low power, tolerant of a brief gap, and the reason many households buy their first UPS. The network operator's advice to protect internet routers from voltage variations with surge protectors applies alongside the UPS.¹ See also will my Wi-Fi work during a power cut?
• Desktop PC and monitor. The classic offline or line-interactive load. A short transfer is acceptable; the value is in the minutes to save and shut down.¹
• Home office with a server or NAS. Here line-interactive is the sensible floor, and online double conversion is the defensible choice, because a restart costs more than the price difference.
• Sensitive or expensive electronics. Double conversion's rebuilt output is the point, not the transfer time.
• High-power loads such as EV charging. Not a UPS job at all. Electricity North West notes that fast chargers "may incur additional costs from us to reinforce the local electricity network"⁹, which is a reminder that big loads interact with the network, not with a plug-in backup box.

One boundary worth drawing firmly: a UPS is for the loads you choose, not the whole house. Deciding what stays on is a planning exercise in itself, and our page on essential loads and backup circuits works through it. Whole-house cover needs a different class of equipment, covered in standby generators and home battery backup.

Price and capacity: costs as sizing and topology increase

Two numbers drive what a UPS costs: how much power it can carry, and which of the three topologies it uses. Capacity rises in steps, and price rises with it; topology adds a second premium on top, because online double conversion electronics run all day and are built accordingly. Published UK prices for specific models are not given here, because UPS pricing in practice is quoted by the supplier or retailer at the point of sale, and any figure a household sees should be read with its VAT position and date in mind. Our page on backup power costs compared puts UPS costs alongside generators, power stations and batteries.

Sizing starts with the connected load. UK single-phase household appliances are rated at not more than 300 V under the IEC 60335-2-40 standard, with 600 V the limit for other appliances including DC-supplied and battery-operated ones¹⁰, which frames the electrical world a domestic UPS lives in. The practical steps are the same whatever the topology:

1. List the equipment the UPS will carry and note each item's power draw.
2. Add the draws together and allow headroom above the total, because a UPS run at its limit delivers less runtime and shorter battery life.
3. Check the runtime the maker states at that load, not at the rating, since runtime falls as load rises.
4. Match the topology to the load's tolerance, using the transfer time logic above.

The battery chemistry behind the runtime is its own trade-off, covered in lithium-ion vs lead-acid UPS. And if the equipment list grows to include heating or medical devices, the answer is usually not a bigger UPS but a different category of backup, starting with keeping heating running in a power cut and help during a power cut if you rely on medical equipment.

How home batteries and solar generators compare with UPS units

The line between a UPS and a home battery has narrowed. A home battery stores electricity, usually from solar: Solar Energy UK describes batteries as "a way for homeowners to store excess electricity produced from their solar system", and notes that with a rooftop solar system and a home battery, "as much power produced onsite is used as possible".¹² Energy Saving Trust adds that home battery storage is usually combined with solar panels, a smart time-of-use tariff, or both.¹³ Which? lists the charging sources as excess generation, the mains supply, or off-peak grid electricity on a time-of-use tariff.¹⁴

The overlap is in the switchover. AlphaESS markets the SMILE-G3's sub-20 millisecond switch-over explicitly as UPS-grade backup, alongside 200% peak overloading and scalable capacity of up to 60.5 kWh³, and Fronius's GEN24 firmware releases include backup functions and improved backup switching time⁷. A home battery with a fast changeover can therefore do a UPS's job, and then keep going for hours rather than minutes. The trade-offs are cost, installation and complexity: a plug-in UPS is a ten-minute purchase, while a battery is a fitted system. Under the Warm Homes scheme rules, domestic batteries can only be installed where they complement existing or new solar PV¹⁵, which shapes what grant-funded installations look like.

Portable power stations sit at the other end of the same spectrum. Jackery's customer stories include a Solar Generator 1000 v2 owner using it for camping and storm backup, adding a 200W solar panel during the year¹⁶. These units have no AVR and no automatic switchover, so they are not UPS substitutes for a running computer, but they outlast any UPS by hours. The comparison is worked through in UPS vs portable power station and home battery vs portable power station.

For a household thinking about energy independence, the honest summary is this: a UPS removes dependence on the grid for seconds, a portable power station for hours, and a solar-plus-battery system for a meaningful share of daily use. Energy Systems Catapult's Living Lab includes 72 homes with solar panels, batteries and EVs¹⁷, a picture of where the top of that ladder leads. Off-peak charging windows, often overnight between 00:30 and 04:30¹⁸, are what make the battery route economic.

Retired models and end-of-service implications for owners

UPS models are discontinued on a rolling schedule, and the dates are published by the makers. Schneider Electric's APC range offers several worked examples. The Smart-UPS RT 2000VA 230V stopped accepting offers on 25 August 2023¹⁹. The Easy UPS BV1000I, a 1000VA unit with AVR, is stated for discontinuation on 31 December 2026, with end-of-service on 31 December 2028, the last date for maintenance services such as repair and spare parts⁴. The Back-UPS BX1200MI-GR is stated for withdrawal from production on 13 August 2027, with after-sales service continuing to the end of the product's service life²⁰. Even surge protectors follow the pattern: the SurgeArrest Essential PME6U2B-UK stopped accepting offers on 8 April 2025²¹.

For an owner, the sequence to watch is the battery first and the service date second. A UPS with a healthy battery and years to run before end-of-service is an asset; the same unit past its service date is a consumable. If you are buying second-hand or inheriting a unit, checking the model's status on the maker's site takes minutes and tells you whether spare batteries will still be available when yours needs replacing. Our pages on APC and Eaton units, and on maintaining and testing backup power equipment, pick up the practical side of ownership.