A PSU (power supply unit) is the component that converts alternating current from the wall outlet into regulated direct current at the low voltages computer parts use, chiefly +12 V, +5 V and +3.3 V. In computer hardware, the PSU is part of the power and cooling subsystem: it determines how much power a system can draw, how much of it is wasted as heat and how well components are protected when something goes wrong.
A PSU is rated by its continuous DC output power in watts, not by what it pulls from the wall. A 750 W unit can deliver 750 W to the components, while its input draw is higher by the amount lost to conversion inefficiency.
How a switch-mode PSU converts power
Every modern computer PSU is a switch-mode power supply: instead of a heavy low-frequency transformer, it chops the rectified mains voltage at tens or hundreds of kilohertz, which allows a small, light high-frequency transformer. The conversion runs in several stages.
- An EMI filter suppresses electrical noise in both directions between the PSU and the mains.
- A bridge rectifier turns the 100-240 V AC input into pulsating DC.
- Active power factor correction (PFC) boosts this to roughly 380-400 V DC and shapes the input current to follow the voltage waveform.
- Bulk capacitors store energy and determine the hold-up time during brief outages.
- The main converter, usually an LLC resonant design in quality units, drives the transformer that isolates and steps down the voltage.
- Synchronous rectification produces 12 V, and DC-to-DC converters derive the 5 V and 3.3 V rails from it.
The ATX specification defines the voltages, their permitted tolerance and the maximum ripple, as shown below.
| Rail | Tolerance | Maximum ripple | Main consumers |
|---|---|---|---|
| +12 V | ±5% | 120 mV | CPU and GPU voltage regulators, fans |
| +5 V | ±5% | 50 mV | SATA drives, USB ports |
| +3.3 V | ±5% | 50 mV | M.2 SSDs, chipset logic, PCIe slots |
| +5 VSB | ±5% | 50 mV | Standby power for wake functions |
| −12 V | ±10% | 120 mV | Legacy serial interfaces |
Today almost the entire load sits on +12 V, because the CPU and GPU use their own voltage regulators that start from 12 V. A PSU with a weak 12 V capacity is therefore weak overall, regardless of its label.
Efficiency and 80 PLUS ratings
PSU efficiency is the ratio of DC output power to AC input power, and the difference leaves the unit as heat. The 80 PLUS program certifies minimum efficiency at fixed load points. The values below apply to 115 V internal, non-redundant units.
| Level | At 10% load | At 20% load | At 50% load | At 100% load |
|---|---|---|---|---|
| 80 PLUS | Not specified | 80% | 80% | 80% |
| Bronze | Not specified | 82% | 85% | 82% |
| Silver | Not specified | 85% | 88% | 85% |
| Gold | Not specified | 87% | 90% | 87% |
| Platinum | Not specified | 90% | 92% | 89% |
| Titanium | 90% | 92% | 94% | 90% |
The practical gap is modest per watt but adds up. A 400 W DC load draws about 444 W from the wall at 90% efficiency and about 471 W at 85%, so the Gold unit wastes 44 W instead of 71 W. Operating on 230 V mains typically adds one to two percentage points. The Cybenetics ETA and LAMBDA ratings offer a finer-grained alternative that also grades fan noise.
Protection circuits and transient response
A quality PSU contains independent protection circuits that shut it down before a fault damages the components. Cheap units often omit some of them or set thresholds so high that they never trigger.
| Protection | Trigger | Purpose |
|---|---|---|
| OCP (over-current) | Current on a rail exceeds its limit | Prevents cable and connector overheating |
| OVP (over-voltage) | Rail voltage rises too high | Protects sensitive chips |
| UVP (under-voltage) | Rail voltage drops too low | Avoids unstable operation and data errors |
| OPP (over-power) | Total output exceeds the rating | Limits overall stress on the unit |
| OTP (over-temperature) | Internal temperature too high | Prevents component failure from heat |
| SCP (short-circuit) | Near-zero resistance on an output | Stops a catastrophic fault instantly |
Modern graphics cards produce power excursions, current spikes lasting microseconds to milliseconds that can far exceed their average draw. The ATX 3.x specification requires units to tolerate brief excursions up to twice the rated power and introduced the 12V-2×6 connector, which supplies up to 600 W to a graphics card through one cable.
Sizing and choosing a PSU
The correct PSU size covers the sustained peak draw of the system with a reserve of roughly 20 to 40 percent. Efficiency curves are flat between about 30% and 80% load, so the common belief that a unit must run at exactly 50% load is an exaggeration. Oversizing a PSU far beyond need mainly increases cost and lowers efficiency at idle.
| System class | Typical peak DC draw | Recommended rating | Note |
|---|---|---|---|
| Office PC with integrated graphics | 80-150 W | 300-450 W | Low-load efficiency matters most |
| Mid-range gaming PC | 350-500 W | 650-750 W | Gold efficiency is a sensible minimum |
| High-end gaming PC | 650-800 W | 1,000 W | ATX 3.x with a native 12V-2×6 cable |
| Multi-GPU workstation | 1,200 W or more | 1,600 W | Check the circuit limit of the wall outlet |
Beyond wattage, buyers compare form factor (ATX or the smaller SFX), cable modularity, warranty length and fan behavior, such as a zero-RPM mode at low load. A long warranty, often 10 to 12 years, usually signals higher-grade capacitors.
Common mistakes and failure symptoms
Most PSU-related failures come from wrong sizing, improper cabling or aging capacitors. The table links common symptoms to their causes.
| Symptom or mistake | Cause | Fix |
|---|---|---|
| Shutdown during gaming spikes | OCP or OPP triggered by GPU excursions | Use an ATX 3.x unit with adequate headroom |
| Dead drives after a PSU swap | Modular cables reused from another brand | Use only the cables supplied with the unit |
| Melted GPU power connector | Connector not fully seated | Insert fully and avoid sharp bends near the plug |
| Coil whine under load | Magnetostriction in inductors | Usually harmless; replace if severe |
| Random reboots in an old system | Degraded capacitors and rising ripple | Replace the PSU rather than repair it |
The short version
A PSU is the power adapter built into a desktop computer. The wall socket provides high-voltage alternating current, but computer chips need low, perfectly steady direct current. The PSU makes that conversion, much like a water treatment plant that turns river water into clean, pressure-controlled tap water.
A good PSU delivers enough power, wastes little of it as heat and switches off safely when something goes wrong. A poor one can cause crashes and, in the worst case, damage every part it feeds.
Where it makes a difference
A gamer upgrades to a graphics card rated at 450 W and keeps an older 750 W Bronze PSU. The processor draws up to 200 W and the rest of the system about 75 W, so the sustained total is about 725 W, leaving only 3% headroom. During demanding scenes, millisecond spikes from the graphics card trigger over-current protection and the PC switches off without warning.
The gamer replaces the unit with a 1,000 W ATX 3.1 Gold PSU that has a native 12V-2×6 cable. The same load now runs at about 73% of capacity, the spikes stay within the specification and the shutdowns stop. At 90% instead of roughly 84% efficiency at that load, the wall draw falls from about 863 W to about 806 W, which also reduces heat inside the case.