On this page
Two ways to make AC from a battery
A UPS battery supplies steady DC. To produce AC, the inverter switches that DC back and forth. How finely it switches determines the shape of the output.
Pure sine wave
The inverter switches tens of thousands of times per second (pulse width modulation) and a filter smooths the result into a curve that closely follows a utility sine wave. For a 120 V system the voltage rises smoothly to a peak of about 170 V, falls through zero, and reaches about minus 170 V, 60 times per second. Good units keep total harmonic distortion (THD) in the low single digits of percent under linear loads.
Simulated (stepped) sine wave
The inverter switches between only three levels: full positive, zero, and full negative. Each half cycle is a flat-topped rectangular pulse with a pause at zero in between. The pulse width and height are chosen so the RMS voltage comes out near 120 V, which is what a simple voltmeter reads.
A bit of arithmetic shows the trade-off. For a three-level wave with peak voltage Vp that is "on" for a fraction d of each cycle:
If the designer wants the same peak as a sine wave (170 V) and 120 V RMS, then d = (120 / 170)² ≈ 0.50: the output sits at zero for half of every cycle. Designs that use a lower peak must stay "on" longer. Either way, the wave contains large harmonics at 3, 5, 7 and higher multiples of 60 Hz, typically putting THD in the tens of percent, and its edges change voltage almost instantly instead of over a few milliseconds.
| Property | Pure sine | Simulated sine |
|---|---|---|
| Shape | Smooth curve | Flat steps with zero dwell |
| Peak voltage (120 V system) | about 170 V | Varies by design, often at or below 170 V |
| Harmonic distortion | Low (often under 5% THD) | High (tens of percent) |
| Voltage edges | Gradual | Near-instant jumps |
| Where you see it | Most business line-interactive, all online, many consumer units | Most budget standby and consumer line-interactive units |
| Relative cost | Higher | Lower |
Only on battery
Standby and line-interactive units pass the utility waveform through while utility power is present. A simulated sine UPS therefore only exposes your equipment to the stepped wave during outages, deep sags and self-tests. Online units always output their own inverter waveform, which is why they are always pure sine.
Why active PFC power supplies are the main concern
Most modern desktop and server power supplies have active power factor correction. The input stage is a boost converter that deliberately draws current in proportion to the instantaneous input voltage, so that current tracks a sine wave. It is designed and tested around a sine wave input. A stepped wave upsets it in several ways:
- Edge current spikes. Every power supply has small capacitors across its input in the EMI filter. Capacitor current is proportional to how fast voltage changes, so a near-vertical voltage step produces a sharp current spike. The UPS sees these spikes as high peak current and may report overload, even when the average watts are well within rating.
- The zero dwell. For a large fraction of each cycle, input voltage is zero and the PFC stage cannot draw anything. The supply must run from its bulk capacitor during each pause and then draw harder during the flat top.
- Input sensing confusion. PFC controllers estimate input voltage to set their behavior and to detect brownouts. A flat-topped wave can look like a fault to some controllers, which then shut the supply down as a protective measure.
The practical symptoms are consistent across many user reports: the PC turns off at the moment of transfer to battery, or during a UPS self-test; the PSU buzzes or whines on battery; or the UPS shows an overload alarm only when on battery. These problems are more likely at higher loads. Plenty of active PFC supplies run happily on stepped output, particularly at light load, and some UPS makers state that their simulated sine products support active PFC loads up to a stated level. There is no universal rule, so treat compatibility as model-specific.
Our analysis: the problem hides until the worst moment
The insidious part of a PFC incompatibility is timing. On utility power everything works, so the setup looks fine for months. The incompatibility only appears during an outage, under whatever load the PC happens to be running, which is exactly when you are relying on it. Idle desktops at 80 W may ride through a test, then the same machine at 400 W during a game or render drops instantly. Our rule: if you test a simulated sine UPS with a PFC load, test it at the highest realistic load, not at idle. A pass at idle proves very little.
Which loads care, and how much
| Load | Behavior on simulated sine | Recommendation |
|---|---|---|
| Modem, router, ONT, switch (external DC adapter) | Usually fine; small adapters tolerate stepped input | Simulated OK |
| Laptop charger, phone charger | Usually fine; may run slightly warm or whine | Simulated OK |
| Office PC, older or non-PFC supply | Usually fine | Simulated OK, pure sine preferred |
| Desktop PC with active PFC, moderate load | Model dependent; may shut off at transfer | Pure sine recommended |
| Gaming PC or workstation at high load | Highest risk of shutdown or overload alarms | Pure sine |
| NAS, server, storage, redundant PSUs | Model dependent; consequences of a drop are severe | Pure sine |
| Induction motors: fans, pumps, furnace blowers, aquarium pumps | Run hotter and noisier; harmonics waste energy as heat; startup may struggle | Pure sine |
| Audio amplifiers, studio gear | Audible hum or buzz on battery | Pure sine |
| Clocks and timers that count AC cycles, dimmers, some appliances with electronic controls | Can drift, flicker or misbehave | Pure sine |
| Medical devices (CPAP, oxygen concentrators and others) | Device makers often specify sine wave power or a particular backup method | Follow the device maker's guidance; see UPS for CPAP |
| Laser printers, heaters | Should not be on battery outlets regardless of waveform | Surge-only outlet |
Why motors suffer
An induction motor responds mainly to the 60 Hz fundamental. The 3rd, 5th and 7th harmonics in a stepped wave produce currents that create no useful torque but heat the windings and the core. Some harmonics even produce torque that opposes rotation. The result is extra heat, extra noise, and higher current drawn from the UPS for the same mechanical output. For a sump pump or furnace blower, which may run for long stretches during an outage, that matters. See UPS for sump pump and UPS for gas furnace.
Why audio gear hums
Fast edges couple into audio circuits through power supply transformers and ground paths, producing a buzz at multiples of 60 Hz. It is harmless but obvious, and it only occurs on battery.
Diagnosing a waveform problem you already have
If your PC turns off when the power fails even though the UPS stays on, work through these checks before blaming the battery:
- Check the UPS spec sheet. If the output waveform on battery is listed as simulated, stepped or modified sine, and the PC has an active PFC supply, the waveform is a prime suspect.
- Check the load. Use the UPS display or a plug-in meter. If the load is near the watt rating, the cause may be overload instead; see VA vs watts.
- Test at idle and at load. Run the UPS self-test (or switch off the upstream outlet or breaker) once with the PC idle, then again under a heavy load. Shutdown only under load points to waveform or capacity.
- Check battery health. A weak battery can collapse under load and look like a waveform issue. See UPS shuts off during outage.
- Swap if needed. If the PC fails on a stepped wave under load with a healthy battery, the reliable fix is a pure sine UPS. Firmware or settings rarely change the waveform.
When simulated sine is the smart buy
Pure sine is not always necessary. A simulated sine unit makes sense when:
- the load is a modem, router, ONT, small switch or VoIP adapter powered by external adapters (see UPS for modem and router);
- the load is a laptop dock, small office PC with a known-tolerant supply, or a set-top box;
- you need the cheapest possible ride-through for a few seconds of flicker, and you have verified compatibility under realistic load.
For a wider view of matching a UPS to a gaming rig, see sizing a UPS for a gaming PC. For harmonics in general, see harmonics and THD.
Bottom line
The waveform only matters on battery, but that is precisely when the UPS has to work. For any modern desktop, NAS, server, motor or audio load, buy pure sine. Keep simulated sine for small network gear and chargers, and if you must use it with an active PFC PC, prove it with a test at full realistic load.
Frequently asked questions
Will a simulated sine wave UPS damage my PC?
Damage is unlikely for a brief outage. The more common outcome with an active PFC power supply is that the PC shuts off when the UPS transfers to battery, or the supply makes a buzzing noise while on battery. That defeats the purpose of the UPS, which is why pure sine is recommended for modern desktops, especially gaming PCs near the upper load range.
Is modified sine wave the same as simulated sine wave?
In practice, yes. Modified sine wave, simulated sine wave, stepped approximation and quasi-square wave all describe outputs built from flat voltage steps rather than a smooth curve. Exact step patterns differ between products, which is one reason compatibility varies from model to model.
Does pure sine wave output give longer runtime?
Not inherently. Runtime depends on battery energy, inverter efficiency and load. Some motor loads draw extra current on stepped output and waste it as heat, which can reduce runtime a little, but for electronics the difference is usually small. Choose the waveform for compatibility, not runtime.
How do I know if my power supply has active PFC?
Check the PSU label or spec sheet for active PFC, or look at the input voltage range: a full-range input such as 100 to 240 V with a power factor around 0.9 or higher strongly suggests active PFC. Most desktop PSUs sold in recent years for regions with harmonic limits or efficiency certifications use it.
Can I check the waveform of my UPS myself?
The spec sheet usually states it under output waveform on battery. Measuring it safely requires an oscilloscope with a properly rated isolated or differential probe; do not connect a regular scope probe to mains. A plug-in power quality monitor that reports THD is a safer indirect check.
Sources and further reading
- IEC 62040-3, Uninterruptible power systems (UPS): Method of specifying the performance and test requirements (output waveform classification)
- IEEE Std 519, Recommended Practice and Requirements for Harmonic Control in Electric Power Systems
- IEC 61000-3-2, Limits for harmonic current emissions (equipment input current up to 16 A per phase)
- ENERGY STAR: Uninterruptible Power Supplies