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One feature separates them: AVR
A standby UPS and a line-interactive UPS look alike on a shelf and behave alike during a clean blackout. Both pass utility power to the outlets in normal operation, both keep a battery charged, and both start an inverter when power fails. The difference is the band of "imperfect but present" power in between.
- Standby (offline): utility power goes through a surge suppressor and filter straight to the outlets. If voltage drops below (or rises above) the unit's acceptance window, the UPS switches to battery. There is no in-between.
- Line-interactive: utility power goes through an autotransformer with taps. A moderate sag triggers a relay that selects a boost tap, raising output voltage back toward normal. A moderate swell selects a buck tap. Only when input moves beyond what the taps can correct does the unit go to battery.
That transformer is what you pay extra for. The mechanism is explained in detail in automatic voltage regulation (AVR), and the family of UPS designs, including online double-conversion, is in UPS types explained.
| Feature | Standby (offline) | Line-interactive |
|---|---|---|
| Response to moderate sag (brownout) | Switches to battery | Boosts voltage via AVR, battery untouched |
| Response to moderate swell | Switches to battery (or passes through if within window) | Bucks voltage via AVR |
| Transfer time on outage | Typically a few ms, often up to about 10 ms | Typically a few ms, often slightly faster |
| Waveform on battery | Usually simulated sine | Simulated sine in budget lines; pure sine common in mid and upper lines |
| Typical size range | About 350 to 1000 VA | About 500 VA to 3 kVA |
| Weight and size | Lighter, no regulating transformer | Heavier for the same capacity |
| Battery cycling on a sag-prone grid | Frequent | Rare |
| Relative cost | Lowest | Modestly higher |
What happens in a brownout, step by step
Consider a US home where nominal voltage is 120 V. A neighbor's well pump, your own air conditioner, or an overloaded feeder on a hot afternoon pulls the voltage down to around 100 V for a few minutes. That is a sustained sag (a brownout), covered in detail in brownouts and voltage sags.
Standby UPS
- The unit detects input below its low-voltage threshold. The exact threshold varies by model and some let you adjust sensitivity.
- It opens its input relay and starts the inverter. Your equipment sees a transfer of a few milliseconds and then simulated sine output.
- The battery carries the full load for as long as the sag lasts. A five-minute brownout at moderate load can use a substantial fraction of a small UPS's runtime.
- When voltage recovers, the unit transfers back and starts recharging, which for lead-acid batteries can take hours to reach full charge.
Line-interactive UPS
- The unit detects input below its boost threshold and switches its transformer to the boost tap. You may hear a relay click.
- Output voltage rises back into a normal range. The equipment sees utility-derived power with no transfer and no change of waveform.
- The battery stays at full charge throughout.
- When the sag ends, the relay returns to the normal tap.
If a real outage strikes right after the brownout, the line-interactive unit has its whole battery available. The standby unit may be starting with a partly discharged one.
Worked example: how sags wear a standby battery
This is a qualitative illustration, not a lifetime prediction. Actual battery wear depends on the battery, its temperature, the load and how deep each discharge goes.
Imagine two identical desktop PCs on the same circuit, one on a standby UPS and one on a line-interactive UPS. The circuit sees occasional real outages a few times a year. It also sees sags deep enough to leave a standby unit's window several times a week: compressor starts on a hot day, a space heater in the next room, a weak connection at the panel.
| Event type | Standby UPS | Line-interactive UPS |
|---|---|---|
| Short sags (seconds) | Brief battery discharge each time, followed by recharge | Handled by AVR; no discharge |
| Sustained brownouts (minutes) | Partial to deep discharge, long recharge | Handled by AVR if within boost range |
| Real outages | Battery discharge | Battery discharge |
| Cumulative effect | Hundreds of shallow cycles plus some deep ones | Only the real outages |
Why does this matter? A VRLA lead-acid battery in UPS service ages mostly through calendar time and heat, but every discharge adds wear, and deep discharges add much more than shallow ones. Lead-acid batteries also age faster if they repeatedly sit partly charged, because sulfation sets in on plates that are not fully recharged. A standby unit on a sag-prone circuit spends a lot of time recovering from the last event. In practice, that shows up as a battery that fails its self-test or delivers short runtime sooner than the same battery in a line-interactive unit. For the aging mechanisms, see UPS battery lifespan.
Our analysis: the hidden cost is the surprise flat battery
Battery replacement cost is the obvious penalty, but the more serious one is timing. A standby unit that has been riding out sags all afternoon may have only a fraction of its runtime left when the storm finally takes the power out. If your UPS log shows many short "on battery" events, that is a strong sign that a line-interactive unit (or fixing the cause of the sags) will improve real outage protection, not just battery life.
Transfer time and waveform
On a clean, sudden outage, both designs take a few milliseconds to transfer. Spec sheets for both commonly list figures in roughly the 2 to 10 ms range, and the measured figure depends on where in the AC cycle the failure happens and how the manufacturer defines it. See UPS transfer time for the details.
Waveform is a separate question. Most standby units and many budget line-interactive units output a simulated (stepped) sine wave on battery. Most electronics accept it, but some active PFC power supplies can shut down, click or buzz on simulated sine, particularly at higher loads. If you are protecting a modern desktop PC or anything with a sensitive power supply, look for pure sine wave output, which is far more common in line-interactive lines than in standby ones. Background: pure vs simulated sine wave and active PFC power supplies.
Use cases: matching the type to the load
| Load | Recommendation | Reasoning |
|---|---|---|
| Cable modem, router, ONT | Standby (or a DC mini UPS) | Low draw, tolerant adapters, cost matters more than AVR |
| VoIP adapter or cordless phone base | Standby | Same reasoning as networking gear |
| Desktop PC with active PFC power supply | Line-interactive, pure sine | Avoids needless transfers and waveform issues |
| NAS or home server | Line-interactive, pure sine, with USB shutdown | Battery health and clean shutdown both matter |
| Home theater, game console | Line-interactive | AVR prevents nuisance transfers; consoles usually fine on either waveform but check |
| Any load on a circuit with frequent sags | Line-interactive (or online if severe) | Protects battery and preserves runtime |
| Tight budget, stable utility power, one small device | Standby | On clean power the difference is small |
For networking gear specifically, including PoE and fiber ONTs, see UPS for modem and router. For desktops, UPS for home office covers sizing and outlet planning.
Bottom line: router and networking gear
A standby UPS is a perfectly good choice. The load is small and tolerant, the unit is cheap and compact, and occasional battery use on a sag costs little. Spend the difference on runtime instead.
Bottom line: desktop PC, NAS, or home office
Choose line-interactive with pure sine wave output. AVR keeps the battery fresh for real outages, and pure sine avoids problems with active PFC power supplies. The price gap at common home sizes is usually modest compared with one early battery replacement.
Before you blame the UPS type
A UPS that switches to battery many times a day is reporting a problem, not causing one. Before you replace a standby unit, consider:
- Sensitivity settings. Many units offer high, medium and low sensitivity. Lower sensitivity widens the acceptance window and can cut nuisance transfers, at the cost of passing slightly rougher power.
- Shared circuits. Laser printers, space heaters and window air conditioners on the same circuit can pull voltage down every time they cycle. Moving either load can help.
- Wiring faults. A loose neutral or a failing connection can cause sags that come and go. If you see flickering lights elsewhere too, call an electrician. See site wiring faults and grounding.
Our troubleshooting guide, UPS keeps switching to battery, walks through the checks in order.
Never put heaters or laser printers on battery outlets
Space heaters, hair dryers and laser printers draw large surges that can overload any small UPS, standby or line-interactive. They also cause the very sags that make a UPS transfer. Plug them into a separate circuit, never into a battery-backed outlet.
Frequently asked questions
Is a standby UPS good enough for a modem and router?
Usually yes. Modems, routers and ONTs draw little power, run from external adapters that tolerate brief interruptions and simulated sine wave, and rarely sit on circuits with heavy sags. A small standby UPS, or a DC mini UPS, keeps internet up through short outages at low cost. See our guide to UPS for modem and router setups for runtime planning.
How can I tell if a UPS is standby or line-interactive?
Look for AVR, boost, buck or voltage regulation in the spec sheet. Line-interactive units list it explicitly and often state an input range they correct without battery. Standby units describe an input voltage window and say nothing about regulation. Desk units under about 600 VA with outlets on top are frequently standby, but check the specs, not the shape.
Why does my UPS click on and off during storms or when the AC starts?
Large motor loads such as an air conditioner compressor cause short voltage sags when they start. A line-interactive unit clicks as its AVR relay switches taps; a standby unit clicks as it goes to battery and back. Occasional clicks are normal. Constant clicking suggests a wiring problem or a heavily loaded circuit worth investigating.
Does line-interactive mean pure sine wave?
No. Topology and waveform are separate features. Many budget line-interactive units produce a simulated (stepped) sine wave on battery, just like standby units. If your PC has an active PFC power supply, look specifically for pure sine wave output in addition to AVR.
Do standby UPS units have a shorter battery life?
The battery is often the same type and size, so on clean power the life is similar. The difference appears on poor power: a standby unit discharges the battery for every out-of-window sag, and each partial discharge adds wear. On a stable grid with rare outages, a standby battery can last as long as one in a line-interactive unit.
Sources and further reading
- IEC 62040-3, Uninterruptible power systems (UPS): Method of specifying the performance and test requirements
- UL 1778, Standard for Uninterruptible Power Systems
- Battery University (Cadex Electronics): lead-acid cycling and aging
- IEEE Std 1188, Recommended Practice for Maintenance, Testing, and Replacement of Valve-Regulated Lead-Acid (VRLA) Batteries for Stationary Applications