Power Supplies and Power

What a power supply does

The electricity from a wall outlet is alternating current (AC) at roughly 230 V in Europe and most of the world, or 120 V in North America. Chips work on low-voltage direct current (DC). The power supply unit (PSU) converts one into the other: it rectifies and filters the AC, then uses a switching regulator to produce steady DC outputs, and it shuts down when something goes wrong. A desktop PSU delivers three main voltages, plus two small ones:

• +12 V: by far the largest share. The processor, graphics card, fans and drive motors all run from it.
• +5 V: USB ports, the electronics of Serial ATA (SATA) drives, and older components.
• +3.3 V: some chips on the motherboard and expansion slots, such as M.2 slots.
• +5 V standby (5VSB): stays on whenever the PSU is plugged in and switched on at the back, even when the computer is "off". It powers the power button circuit, wake-on-LAN and USB charging while the system is off.
• −12 V: a legacy output, barely used today.

The processor itself runs at around 1 V. The motherboard's voltage regulator module (VRM) converts 12 V to that core voltage, right next to the socket.

Laptops, small desktops and many other devices use an external power adapter (the "brick") that does the same AC-to-DC conversion outside the case, often delivering around 19–20 V for the laptop's own regulators, or power over USB-C with USB Power Delivery. See Laptops and Mobile Devices.

Wattage and sizing

A PSU's wattage is the maximum continuous DC power it can deliver to the components, in watts (W). Power is voltage times current (W = V × A), and the label also lists the maximum current per output; on modern units nearly all the wattage is available on the 12 V output. A PSU does not push its full wattage into the computer: the components draw what they need, and a 750 W unit in an office PC that uses 80 W simply delivers 80 W.

To size a PSU, add up the maximum draw of the components, not the average, and add headroom. The graphics card and processor dominate; their datasheets list the power they can reach under full load, which can be higher than the processor's thermal design power (TDP). Headroom covers short load spikes, future upgrades and aging, and keeps the PSU in the middle of its range, where it runs most efficiently and quietly:

# Estimated maximum draw per component, in watts (from datasheets)
parts = {
    "Processor": 125,
    "Graphics card": 220,
    "Motherboard": 50,
    "Memory (2 modules)": 10,
    "NVMe SSD": 8,
    "Hard drives (2)": 18,
    "Case fans (4)": 12,
    "USB devices": 15,
}
STANDARD_SIZES = [450, 550, 650, 750, 850, 1000]

total = sum(parts.values())
target = total * 1.3                  # 30% headroom
psu = next(size for size in STANDARD_SIZES if size >= target)

for name, watts in parts.items():
    print(f"{name:<20}{watts:>4} W")
print(f"{'Total':<20}{total:>4} W")
print(f"With 30% headroom: {target:.0f} W -> choose a {psu} W power supply")

Processor            125 W
Graphics card        220 W
Motherboard           50 W
Memory (2 modules)    10 W
NVMe SSD               8 W
Hard drives (2)       18 W
Case fans (4)         12 W
USB devices           15 W
Total                458 W
With 30% headroom: 595 W -> choose a 650 W power supply

The component figures are illustrative; use the datasheets of the actual parts, and the graphics card maker's recommended PSU size, which already includes a margin. Planning a whole build or upgrade around this sum is covered in Choosing and Upgrading.

Efficiency and 80 PLUS

No conversion is perfect. Efficiency is the DC power delivered divided by the AC power drawn from the wall; the difference turns into heat inside the PSU, which its fan must remove. A more efficient PSU uses less electricity, runs cooler and often quieter:

dc_load = 400   # watts the components draw from the power supply

for rating, efficiency in [("80 PLUS Bronze", 0.85), ("80 PLUS Gold", 0.90), ("80 PLUS Titanium", 0.94)]:
    from_wall = dc_load / efficiency
    heat = from_wall - dc_load
    print(f"{rating:<17} {efficiency:.0%}: {from_wall:5.1f} W from the wall, {heat:4.1f} W heat")

80 PLUS Bronze    85%: 470.6 W from the wall, 70.6 W heat
80 PLUS Gold      90%: 444.4 W from the wall, 44.4 W heat
80 PLUS Titanium  94%: 425.5 W from the wall, 25.5 W heat

80 PLUS is a certification program for PSU efficiency. A unit is tested at 20%, 50% and 100% of its rated load (Titanium also at 10% load) and must reach a minimum efficiency at each point; higher levels require more. The thresholds for standard desktop PSUs tested at 115 V are:

Titanium also requires 90% at 10% load. At the time of writing, a Ruby level (2025), aimed at data-center power supplies, sits above Titanium. At 230 V, conversion is a little more efficient, and the thresholds for that test are a few percentage points higher. Efficiency peaks around half load, which is one more reason for headroom. Cybenetics is a newer, independent rating program that also measures noise.

Connectors

Each cable from the PSU ends in a connector with its own shape and keying, so it only fits the right way into the right socket.

12V-2x6 is the revised version of 12VHPWR, introduced with the ATX 3.1 specification. Its sense pins are shorter, so the card only draws full power when the plug is seated completely; a plug that is not fully inserted can overheat at these currents. Inside the cables, ATX uses fixed wire colors: yellow for +12 V, red for +5 V, orange for +3.3 V, black for ground, purple for 5 V standby, green for the power-on signal and gray for "power good", the signal with which the PSU tells the motherboard that its outputs are stable.

Modular designs, input voltage and redundancy

Modular, semi-modular and non-modular

In a non-modular PSU, every cable is permanently attached; the unused ones have to be tucked away in the case. A modular power supply has sockets on the unit, so you connect only the cables you need, which improves airflow and makes building and cable management easier. A semi-modular PSU has the always-needed cables (24-pin and CPU) fixed and the rest detachable.

Input voltage

Nearly all current PSUs and laptop adapters are auto-switching (full-range): they accept anything from about 100 to 240 V at 50 or 60 Hz, so they work in any country with the right plug. Older and some cheap PSUs had a manual 115/230 V selector switch on the back. Set to 115 V on a 230 V outlet, such a unit can fail immediately; set to 230 V on a 115 V outlet, it will not start properly. Check the label before plugging in a device from another region.

Redundant power supplies

Servers and network equipment often have a redundant power supply: two (or more) hot-swappable PSU modules, each able to carry the full load on its own. If one fails, the other takes over without interruption, the server raises an alert, and the failed module is swapped while the system runs. For real redundancy, the two modules are connected to separate circuits (often called A and B feeds), ideally through separate uninterruptible power supplies (UPSs), so that a tripped breaker does not take out both.

Protecting equipment

Surge protectors

A surge protector diverts short voltage spikes, for example from lightning strikes nearby or large equipment switching, away from the connected devices. Its protection is rated in joules (how much energy it can absorb over its life: higher is better) and in clamping voltage (the voltage at which it starts to divert: lower is better). The protective components wear with every surge, so a surge protector does not last forever; many have an indicator that goes out when protection is gone. An ordinary power strip only adds outlets and offers no surge protection. Neither helps during a power outage.

Uninterruptible power supplies

An uninterruptible power supply (UPS) contains a battery and an inverter, and keeps connected equipment running for minutes when the mains power fails: long enough to ride out a short outage or to shut servers down cleanly. A USB or network connection lets the UPS tell the computer to shut down on its own before the battery runs out. Most UPSs also include surge protection. There are three designs:

UPSs are rated in two numbers. Volt-amperes (VA) is the apparent power: volts times amps. Watts (W) is the real power the equipment uses. For computer loads the watt rating is lower than the VA rating (VA × power factor), and it is the watt rating that must cover your load, with margin:

load_w = 450 + 40 + 15      # computer, monitor, router
needed_w = load_w * 1.25    # keep 25% margin

ups_models = [("UPS A", 1000, 0.6), ("UPS B", 1500, 0.6), ("UPS C", 1500, 0.9)]

print(f"Load {load_w} W, with margin {needed_w:.0f} W")
for name, va, power_factor in ups_models:
    watts = va * power_factor
    verdict = "enough" if watts >= needed_w else "too small"
    print(f"{name}: {va} VA x {power_factor} = {watts:4.0f} W  {verdict}")

Load 505 W, with margin 631 W
UPS A: 1000 VA x 0.6 =  600 W  too small
UPS B: 1500 VA x 0.6 =  900 W  enough
UPS C: 1500 VA x 0.9 = 1350 W  enough

Runtime is a separate question: the lighter the load, the longer the battery lasts, and the maker publishes runtime charts per load. UPS batteries wear out, typically after a few years, and a UPS usually signals this with a self-test warning.

Power problems and symptoms

Power problems are common tickets, and the symptoms point in different directions:

• Nothing happens at all: no fans, no lights. Check the simple things first: the outlet (test it with another device), the power cable, the switch on the back of the PSU, a surge protector that has tripped, in the UK the fuse in the plug, and, after work inside the case, whether the 24-pin and front-panel power button cable are seated.
• Fans spin but the computer does not start: could be the PSU, but also memory, the processor or the motherboard. A missing CPU power cable is a classic cause after a repair.
• Random restarts or shutdowns, especially under load: an undersized or aging PSU, or overheating. Note what the user was doing when it happened.
• Burning smell, smoke, or a loud pop: switch off and unplug immediately. Do not switch it on again to "see what happens"; replace the PSU and check the other components.
• Loud fan, whine or buzzing from the PSU: a failing fan or components. A PSU whose fan has stopped will overheat.

A PSU tester, or a multimeter used by someone trained to use it on the connectors (never inside the unit), confirms whether the outputs are within tolerance. The step-by-step approach is on Hardware Troubleshooting, and the safety rules for working with power are on Safety and Handling.