On this page
- Most likely cause: the UPS is doing its job, a little too often
- A field guide to UPS sounds
- Diagnostic flow: safest checks first
- Symptom, likely cause and fix
- Why voltage that "looks fine" still triggers clicks
- Brand notes (behavior varies, check your manual)
- Repair, replace the battery, or replace the UPS?
- Frequently asked questions
Most likely cause: the UPS is doing its job, a little too often
The single most common noise complaint is a click, sometimes followed by a short beep, that repeats several times an hour. That is a mechanical relay inside the UPS switching the load between utility power and the inverter, or switching the transformer between its normal, boost and trim taps. Each click is the UPS reacting to the input voltage crossing one of its thresholds.
Occasional clicks are expected. Frequent clicks mean the input is sitting right at the edge of what the UPS accepts. The fixes, in order of how often they work:
- Move the UPS to a different circuit, ideally one without a refrigerator, air conditioner, laser printer, space heater or vacuum on it. Big motor and heater loads pull the voltage down each time they start.
- Lower the input sensitivity (often a button sequence on the front panel or a setting in the vendor software). Low sensitivity tolerates a wider voltage and waveform window before transferring.
- Widen the transfer voltage thresholds if your model allows it through its software, staying within the range the manual lists.
- Plug the UPS directly into the wall, not into a surge strip, extension cord or another UPS. See plugging a UPS into a power strip for why that matters.
If clicks are paired with switching to battery and you want the deeper diagnostic for that symptom, go to UPS keeps switching to battery.
A field guide to UPS sounds
Every UPS makes some noise. The useful question is which part is making it and whether that part should be active right now. Here is what each sound usually is.
Relay clicks
Standby (offline) and line-interactive units use relays to transfer the load. You typically hear one click when the UPS goes to battery and another when it returns to line power. Line-interactive units with automatic voltage regulation also click when they engage boost (raising a low voltage) or trim (lowering a high one), with no beep and no battery use. A self-test produces the same click pair, often on a schedule (many units self-test at power-up and then every 7 or 14 days; check your manual).
Online double-conversion units run the inverter all the time, so they rarely click in normal use. A click on an online unit usually means it moved to or from bypass, which is worth checking in the event log because bypass means the load is no longer conditioned.
Transformer hum
Units with an iron-core transformer produce a low, steady hum. Magnetostriction makes the core flex twice per AC cycle, so the fundamental tone is 120 Hz on 60 Hz power (100 Hz in 230 V / 50 Hz regions). The hum is usually faint on line power and noticeably louder on battery, when the inverter drives the transformer hard. Hum also rises with load and when the unit is in boost or trim.
Inverter buzz on battery
Many consumer units produce a simulated (stepped) sine wave on battery. The sharp steps contain harmonics that make both the UPS transformer and the power supplies of connected devices buzz audibly. A buzz that appears only during an outage and stops when the power returns is normal for these models. Pure sine wave units are usually quieter on battery, though not silent.
Fan noise
Small desktop units are often fanless or run a fan only on battery or while charging. Larger towers and rack units typically have fans that run continuously or ramp with load and internal temperature. A steady whoosh is normal. A rattle, grinding, ticking that matches fan speed, or a fan that runs at full speed all the time in a cool room is not.
Coil whine and charger whine
Inductors and transformers in the charger and inverter switch at high frequencies, and a faint high-pitched whine can leak into the audible range. It is common, harmless in most cases, and varies between individual units of the same model. Younger listeners hear it more.
The alarm
Some people describe the piezo beeper as "buzzing," especially a continuous tone. A continuous tone is usually an overload or fault alarm, not a mechanical sound. Use the UPS alarm decoder and the UPS beeping guide to identify the pattern.
Sounds that are never normal
- Crackling, sizzling or snapping: possible arcing at a connector, relay contact or failing component.
- Hissing or bubbling from the battery area: a sealed lead-acid battery venting gas, typically from overcharging or thermal runaway.
- A loud pop, especially followed by silence or smell: a failed capacitor, fuse or semiconductor.
- A buzz that rises steadily over minutes on line power, with the case getting hot.
Stop here if you smell, see or feel any of these
A burning or hot-plastic smell, a rotten-egg (sulfur) smell, a hot or bulging case, visible smoke, or crackling and hissing: unplug the UPS from the wall, shut down and move the connected equipment to another outlet, and keep the unit away from anything flammable in a ventilated spot. Do not open it or keep using it. A swollen or very hot battery can vent hydrogen and acid. See UPS hot or burning smell and swollen UPS battery.
Diagnostic flow: safest checks first
Work through these in order. Stop at the first step that explains the noise.
- Smell and touch test (no tools). With your hand flat on the top and sides, is the case merely warm (normal) or too hot to keep your hand on? Any smell or bulging? If yes, unplug and stop, per the box above.
- Note what the UPS is doing when it makes the noise. On line power or battery? Is a battery, AVR, overload or fault indicator lit? Does the noise coincide with a beep, a fridge compressor starting, or a lamp dimming?
- Read the event log. Vendor software such as PowerChute or PowerPanel, or NUT and apcupsd, record transfers, boost/trim and self-tests with timestamps. Matching clicks to log entries tells you whether they are input events. See the software overview if you have nothing installed.
- Check the load. Hum and fan noise scale with load. If the load is above roughly 80% of the watt rating, the unit will run hotter and louder. Look for anything that should not be on it, especially on battery outlets: laser printers, heaters, hair dryers. See UPS load percentage.
- Swap the circuit. Plug the UPS into a different wall outlet, ideally on another breaker, with nothing else between it and the wall. If the clicking stops, the cause is upstream.
- Remove the load as a factor. Unplug the connected devices and listen. If a buzz disappears, it may be coming from a connected device's power supply rather than the UPS.
- Adjust sensitivity. If transfers are frequent and the voltage readings look roughly normal (about 108 to 132 V on a 120 V circuit), lower the sensitivity one step and observe for a day.
- Check the battery. Run a self-test. A battery that fails, or a fan that never stops after a full day of charging, suggests a battery or charger problem. See UPS not charging and when to replace a UPS battery.
Symptom, likely cause and fix
| What you hear | Likely cause | What to do |
|---|---|---|
| One click, sometimes a beep, when lights flicker | Normal transfer to battery and back | Nothing. This is the UPS working. |
| Click with no beep, no battery use, several times a day | AVR boost or trim engaging | Normal. If constant, check voltage at the outlet and move the UPS off circuits with large motor loads. |
| Clicking every few seconds to minutes | Input voltage hovering at a threshold, loose upstream connection, generator or noisy input | Different circuit, lower sensitivity, remove power strips; see switching to battery |
| Rapid clicking on generator power | Frequency or waveform outside the accepted window | Low sensitivity, larger or inverter generator; see UPS not working with generator |
| Clicking repeatedly at power-up, unit will not settle | Failing battery, overload at start, or internal fault | Disconnect the load and retry; test or replace the battery; see UPS won't turn on |
| Low steady hum on line power | Transformer magnetostriction | Normal. Rubber feet or a solid surface can reduce resonance. |
| Loud buzz only on battery | Stepped inverter waveform driving transformer and device power supplies | Normal on many simulated sine models. Consider pure sine for active-PFC loads. |
| Buzz that is much louder than it used to be on line power | Loose transformer laminations, overload, or failing component | Reduce load; if unchanged and the unit is warm, retire it or contact the maker |
| Fan always at full speed | Hot room, blocked vents, high load, battery not finishing charge | Clear 2 to 3 inches around vents, cut load, check charging |
| Fan rattling or grinding | Worn bearing or debris | Clean vents with the unit unplugged; fan replacement is a service job on most units |
| High-pitched whine | Coil or charger whine | Usually harmless; watch for heat or battery warmth |
| Hissing, bubbling, crackling, popping | Battery venting, arcing or component failure | Unplug immediately and stop using the unit |
Why voltage that "looks fine" still triggers clicks
US utility service is supposed to stay within ANSI C84.1 ranges, roughly 114 to 126 V at the service entrance under normal conditions, with somewhat wider limits allowed. Line-interactive UPS units typically start boosting somewhere in the low 100s and trimming somewhere in the high 120s to low 130s, depending on model and settings. A home that sits at 116 V and dips to 105 V each time the air conditioner starts is perfectly normal for lighting, but it crosses a boost threshold every cycle.
Voltage is not the only trigger. Many units also transfer on waveform distortion, fast transients or frequency drift, which a plug-in voltmeter will not show. That is why the sensitivity setting exists: it changes how strict the UPS is about waveform quality, not only voltage.
Our rule of thumb: count the clicks against the log
Fewer than a handful of logged transfers or AVR events per day is normal behavior. If you see dozens per day, the UPS is wearing its relays and, when transfers are involved, cycling the battery, which shortens battery life. In practice, two changes fix most of these cases: moving off a shared circuit with a big motor load, and dropping sensitivity one step. If neither helps and a plug-in meter shows voltage regularly outside about 108 to 127 V, the problem is likely in the building wiring or utility service, and an electrician or the utility should look at it. Our guide to measuring power quality at home explains how to log it.
Brand notes (behavior varies, check your manual)
- APC (Schneider Electric): on many Back-UPS models the sensitivity is set from the front panel or through PowerChute, with low, medium and high settings. Many Smart-UPS models expose transfer voltages and sensitivity in the display menu or software. The PowerChute and PowerPanel guide covers where these live.
- CyberPower: many consumer line-interactive models allow sensitivity and transfer voltage adjustment through PowerPanel Personal or a front-panel button sequence, and display boost/trim status on the LCD.
- Eaton and Tripp Lite: many models offer sensitivity or a generator-friendly mode in the configuration utility or LCD menu; options and names differ by series.
- Online units in general: fans are louder and run continuously because the inverter is always working. That is by design, not a fault.
For an overview of which lines are line-interactive, standby or online, see the APC, CyberPower and Eaton lineup pages.
Repair, replace the battery, or replace the UPS?
| Situation | Best course |
|---|---|
| Clicks traced to input power, unit passes self-test | Fix the circuit or settings. The UPS is fine. |
| Fan runs constantly, battery 3 or more years old, self-test fails | Replace the battery first. See how to replace a UPS battery. |
| Fan rattling on a rack or tower unit under service contract | Have the fan replaced by the maker or an authorized service provider. |
| Fan rattling on an inexpensive desktop unit | Usually not worth repairing. Replace the UPS. |
| Crackling, popping, smell, scorch marks, or a battery that bulged | Retire the UPS. Recycle the battery properly; see UPS battery recycling. |
| Buzz much louder than when new, with new battery and normal load | Likely an internal component issue. Contact the maker if under warranty; otherwise replace. |
Opening a UPS is not a homeowner repair beyond the battery compartment. Capacitors can hold a charge, and the battery string can deliver very high current into a short even with the unit unplugged.
Reducing normal noise
If the noise is normal but annoying: put the UPS on a solid floor or shelf rather than a hollow desk that acts as a sounding board, keep 2 to 3 inches (5 to 8 cm) clear around vents so the fan does not ramp up, keep the room under about 77°F (25°C), and keep the load moderate. Moving a noisy online unit out of the bedroom or office is often the only real fix. See where to place a UPS.
Frequently asked questions
Why does my UPS click when nothing seems wrong with the power?
Many line-interactive units switch transformer taps (boost or trim) when the voltage drifts a few percent, and each change clicks a relay. Your lights will not visibly flicker for that. Check the event log in the UPS software: entries for AVR boost, AVR trim or brief transfers to battery confirm the UPS is reacting to real, if minor, voltage changes.
Is it normal for a UPS to buzz loudly when running on battery?
A louder hum or buzz on battery is normal for many consumer units, because the inverter drives the transformer with a switched waveform. Simulated sine wave models are usually the loudest. It is a concern only if the buzz is new, much louder than before, comes with heat or smell, or the unit shuts off early.
Can a buzzing UPS damage my computer?
A normal transformer hum does not. What matters is the cause: a UPS cycling rapidly between line and battery, or one with a failing inverter, can deliver interrupted or distorted power and may drop the load. If the buzz accompanies reboots, alarms or overload lights, move the equipment to a known-good outlet while you diagnose.
Why does my UPS fan run even when the power is fine?
Many units run the fan while the battery is charging, when the internal temperature rises, or above a certain load. A fan that runs constantly after the battery has had a day to charge may point to a hot room, a blocked vent, a high load, or a battery that never finishes charging. Fans can also wear and start rattling.
My UPS makes a high-pitched whine. Is that a problem?
A faint whine from inductors (coil whine) or from the charger circuit is common and usually harmless, and some people hear it more than others. It deserves attention if it is new and loud, changes with a rattling or burning smell, or comes with the battery getting warm while the unit is on line power, which can indicate overcharging.
Should I change the UPS sensitivity setting to stop the clicking?
Usually yes, if the clicks are frequent transfers caused by slightly noisy or generator power. Lower sensitivity widens the voltage and waveform window the UPS accepts before switching to battery. Your equipment's power supplies typically tolerate that range well. Avoid lowering it if you protect very sensitive gear and the transfers reflect genuine sags.
Sources and further reading
- IEC 62040-3, Uninterruptible power systems: method of specifying the performance and test requirements
- IEEE Std 1100 (Emerald Book), Recommended Practice for Powering and Grounding Electronic Equipment
- ANSI C84.1, Electric Power Systems and Equipment: Voltage Ratings (60 Hz)
- Manufacturer user manuals for APC Back-UPS and Smart-UPS, CyberPower, Eaton and Tripp Lite units (sensitivity, transfer voltage and fan behavior sections)