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Utility voltage is never exactly 120 V. In the US, ANSI C84.1 Range A allows service voltage between 114 and 126 V for a 120 V nominal system, and real buildings see dips below that when large motors start, during heavy demand, or at the end of long rural lines. A UPS needs some way to decide what to do with voltage that is low or high but still present. AVR is the cheap, effective middle option between "pass it through" and "switch to battery".
How boost and buck work
Inside a line-interactive UPS sits an autotransformer: a single winding with several connection points (taps). Relays choose which tap feeds the output. Because the voltage across a transformer winding is proportional to the number of turns, picking a tap changes the output voltage by a fixed ratio.
- Normal (straight through): input connects to output with no ratio change, or through a 1:1 path.
- Boost: input feeds fewer turns than the output takes from, so the output is stepped up by a fixed ratio.
- Buck (trim): the reverse, so the output is stepped down.
Some models have a second, larger boost step (sometimes called double boost) for deep sags, and some have no buck step at all. The steps are coarse: AVR does not hold the output at exactly 120 V, it nudges it back into a range that equipment is happy with.
Worked example: what a boost step does
Take an illustrative boost ratio of 1.12 (a 12% step) and buck ratio of about 0.89 (the inverse of that step).
| Input | Mode | Calculation | Output |
|---|---|---|---|
| 118 V | Normal | 118 x 1.00 | 118 V |
| 102 V | Boost | 102 x 1.12 | 114 V |
| 92 V | Boost | 92 x 1.12 | 103 V |
| 134 V | Buck | 134 x 0.89 | 119 V |
Notice the 92 V row: a single boost step only brings it up to about 103 V. That is why units define a lower limit below which they stop boosting and transfer to battery, and why some add a second boost step.
Typical thresholds (as ranges)
Every manufacturer picks its own set points, and many let you adjust them in software or through front-panel menus. The figures below are typical ranges seen across 120 V line-interactive units, not a specification for any particular model.
| Input voltage | Typical action |
|---|---|
| Roughly 106 to 127 V | Pass through (normal) |
| Below roughly 100 to 106 V | Boost engages |
| Below roughly 80 to 95 V | Transfer to battery (low transfer point; often adjustable) |
| Above roughly 127 to 133 V | Buck engages, on models that have it |
| Above roughly 135 to 147 V | Transfer to battery (high transfer point; often adjustable) |
For 230 V units the same logic applies, with thresholds scaled to the 230 V nominal. Each threshold also has hysteresis: the unit leaves boost at a slightly higher voltage than the one where it entered, so a voltage hovering on the line does not make the relay chatter on every cycle. The gap is small, so wandering voltage can still produce repeated clicks.
Why AVR saves your battery
Without AVR, a UPS has only two choices for low voltage: pass it through, or go to battery. Standby units choose battery at a fairly high threshold to protect the load. AVR adds a third choice that costs almost nothing.
Worked example: an afternoon of air-conditioner sags
Imagine a home office where the input dips to about 101 V for around 15 seconds each time the central air compressor starts, roughly 25 times on a hot day.
| UPS type | What happens on each sag | Battery events per day | Battery time per day |
|---|---|---|---|
| Standby, low transfer at about 104 V | Transfers to battery, beeps, transfers back | 25 | about 6 minutes |
| Line-interactive with AVR | Boost relay clicks in, output about 113 V | 0 | 0 |
| Online double-conversion | Rectifier compensates, no change at the output | 0 | 0 |
Six minutes of battery use per day sounds trivial, but it is 25 shallow discharge cycles, each followed by recharge, every day all summer, plus 25 sets of beeps and 25 transfers, each with a small transfer gap. Over a season that adds up to thousands of transfers. Cycling and the heat of repeated charging both shorten battery life, and the battery may be partly discharged at the moment a real outage arrives.
Our analysis: AVR mostly protects the battery, not the PC
Most modern computer power supplies accept a wide input range, commonly around 90 to 264 V, so a desktop PC usually keeps running at 100 V without help. That means AVR is rarely what keeps a modern PC alive during a sag. Its real value is economic: it keeps the UPS off battery during routine sags so the battery lasts longer and is full when needed. Where AVR does directly protect the load is with older or simpler equipment (transformer-based gear, motors, some appliances) that tolerates low voltage poorly. If you live somewhere with frequent sags, choose a unit with AVR even for a PC that "doesn't need it".
The clicking: normal or a problem?
Each change of tap is a relay closing, and you can hear it. A few clicks a day, often lining up with appliances starting, is normal. Use this table to interpret what you hear:
| Pattern | Likely meaning | What to do |
|---|---|---|
| Occasional single clicks, no beep | Normal boost or buck engagement | Nothing |
| Click with a short beep or "on battery" message | Transfer to battery and back | Normal for brief events; check logs if frequent |
| Clicking every few seconds for long periods | Input voltage hovering at a threshold, or unstable source such as a generator | Check input voltage history; adjust sensitivity or transfer points |
| Rapid clicking with no input disturbance you can find | Possible fault, wiring problem upstream, or a loose connection | See buzzing or clicking noise |
Most units with a display or software show input voltage and the last transfer cause. Logging that for a day or two, or using a plug-in monitor as described in measuring power quality at home, usually explains frequent clicking quickly.
Sensitivity settings
Many line-interactive units offer a sensitivity setting, often labeled high, medium and low (names vary by brand). It changes how strict the UPS is about what counts as acceptable input before it transfers to battery. Depending on the model it may adjust voltage thresholds, the tolerance for distorted waveforms, the acceptable frequency window, or several of these. Exactly what each level changes is brand-specific, so check the manual or the vendor's knowledge base for your unit.
- High (often the default): transfers readily. Best protection for sensitive loads, most battery events.
- Medium: a compromise, tolerating more distortion or wider voltage before transferring.
- Low: accepts rough power, transfers least. Useful on generators and noisy supplies, as long as the load tolerates it.
Separately, many units let you set the low and high transfer voltages directly. Widening them reduces battery use but passes more extreme voltages to the load. Narrowing them protects fussy loads at the cost of more battery cycling.
AVR and generators
Portable generators, especially conventional (non-inverter) ones, produce voltage that wanders with load and frequency that drifts as the engine speed changes. A line-interactive UPS can respond badly in two ways: the AVR relay chatters between taps as voltage swings, and the frequency or waveform falls outside the acceptable window so the unit refuses the input and stays on battery until it runs flat.
Common fixes, in rough order of effort:
- Set the UPS to low sensitivity (if available) so it accepts a wider frequency and waveform window.
- Widen the transfer voltages within what your load tolerates.
- Lighten or steady the generator load, since voltage regulation is often worst when big loads cycle.
- Use an inverter generator, whose output is usually much cleaner.
- Use an online double-conversion UPS, which regenerates its own output and is the most reliable match for generator power.
The full walk-through is in using a UPS with a generator and, if it is already misbehaving, UPS not working with generator.
AVR compared with other ways to handle voltage
| Approach | How it regulates | Precision | Battery use on sags |
|---|---|---|---|
| Standby UPS | Does not; transfers to battery | None | Every significant sag |
| Line-interactive with AVR | Relay-switched transformer taps | Coarse steps | Only below the boost range |
| Online double-conversion | Rectifier and inverter regenerate output | Tight, continuous | Only when input is outside the rectifier's wide range |
| Power conditioner or voltage regulator | Varies: tap switching, servo, or electronic | Varies by type | No battery |
If low voltage is your main problem and runtime is not, a regulator may be enough; see power conditioners. If sags are severe or very frequent, consider an online unit, and read brownouts and voltage sags to understand the source. Persistent voltage well outside the ANSI range is a matter for your utility or an electrician, not something to solve with equipment alone.
Don't let AVR hide a wiring problem
If the UPS sits in boost all the time on one circuit but not on others, the circuit may have a loose connection or an overloaded shared neutral. Loose connections can overheat. Have an electrician check it rather than relying on the UPS to compensate. See site wiring faults and grounding.
Frequently asked questions
Does a UPS with AVR use the battery during a brownout?
Not while the sag stays within the boost range. The transformer raises the voltage using utility power alone, and the battery keeps charging. If voltage falls below the lowest boost threshold, typically somewhere in the 80 to 95 V range depending on the model and settings, the UPS transfers to battery.
Is a UPS with AVR better than a standby UPS?
For most desktops and network gear, yes. A standby unit has no regulation, so every significant sag becomes a battery event. Line-interactive units with AVR handle the frequent, small disturbances without battery cycling, which means fewer alarms, less battery wear and full runtime saved for real outages.
Is AVR the same as a power conditioner?
They overlap but are not the same. AVR in a UPS makes coarse voltage steps with relays and is mainly a battery-saving feature. A dedicated power conditioner may regulate more finely, filter noise more aggressively or provide isolation, and has no battery. See our power conditioners guide for the differences.
Why does my UPS click every few seconds?
The input voltage is probably sitting right at a boost or buck threshold, so the relay switches in and out as voltage wanders. Check the input voltage reading on the display or in software. Lowering the sensitivity setting, if your model has one, or adjusting the transfer points can stop it. Persistent clicking at normal voltage deserves a closer look.
Does AVR fix low voltage caused by an extension cord?
It can mask it, but it does not fix it. A long or thin cord drops voltage under load and the UPS will boost to compensate, but the cord still heats up and the boost wastes headroom. Plug the UPS directly into a wall receptacle, and if voltage at the wall is persistently low, have the circuit checked.
Sources and further reading
- ANSI C84.1, American National Standard for Electric Power Systems and Equipment: Voltage Ratings (60 Hz)
- IEC 62040-3, Uninterruptible power systems (UPS): Method of specifying the performance and test requirements
- IEEE Std 1100 (Emerald Book), Recommended Practice for Powering and Grounding Electronic Equipment
- IEEE Std 1159, Recommended Practice for Monitoring Electric Power Quality