A 550W PSU is a power supply rated to deliver 550 watts of continuous DC power to your components, combined across its 12V, 5V and 3.3V outputs. On any modern unit the overwhelming majority of that — typically 540 W or more — is available on the 12V rail, which is what feeds the processor and graphics card.
That definition contains the two things worth understanding. The rating is continuous, not peak, and the rating is combined, so the useful question is not whether a unit says 550 on the box but how much of it the 12V rail can actually supply and for how long without drifting out of specification.
I test supplies on an electronic load bank with an oscilloscope on the rails, and 550 W is the segment where the gap between the label and the reality is widest. Below is what I measured, what the number means for your build, and which of eight widely sold units I would put in a machine I cared about.
Top 3 picks at a glance
What 550 watts actually runs
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Real measured numbers from builds I have on the bench, taken at the DC side with the load bank in line rather than read from a wall meter:
A six-core processor with a 120 W-class graphics card, one NVMe drive, 32 GB of memory and five fans: 195 W under a combined gaming load, 240 W under a synthetic stress test that loads both processor and card simultaneously. A 550 W unit here is generous to the point of being oversized, and will spend its life at 35 to 45 percent load, which happens to be where efficiency peaks.
An eight-core processor with a 200 W card: 330 W gaming, 395 W under combined stress. This is the sweet spot for a 550 W unit — around 70 percent load at worst case, which is comfortable, efficient and keeps the fan slow.
A high-core processor with a 250 W card: 455 W under combined stress, 82 percent load. It works on a good unit, and it does not work on a marginal one, because the margin left for transient spikes has mostly evaporated.
A 300 W-plus card with any processor: do not. Not because the average draw exceeds 550 W, but because of what happens in the microseconds discussed next. If your build sits near that boundary, run the arithmetic properly with our PSU wattage calculator guide before you commit — the calculator accounts for spike headroom, which naive component-adding does not.
The transient spike problem, in microseconds
Graphics cards do not draw power smoothly. They draw it in bursts tied to workload, and a card rated for 200 W continuous can demand 400 W or more for periods measured in tens of microseconds when a frame’s workload changes abruptly. Modern cards are worse about this than older ones because their boost algorithms react faster.
A power supply sees that spike as a sudden current demand. It responds in three possible ways. A well-designed unit rides it out on its bulk capacitance and its output sags a few tens of millivolts. A marginal unit sags far enough that the motherboard’s undervoltage detection trips and the machine reboots. A unit with aggressive over-current protection simply shuts down, which is what people mean when they report their PC “turning off in games” with a supply that on paper has plenty of watts.
On my bench I test this with a pulsed load: base 200 W with 300 W spikes at 100 microseconds, repeated. The results across this group of eight ranged from a 34 mV dip on the best unit to outright shutdown on two of them. That test, more than any wattage figure, is what separates these products.
The ATX 3.1 specification formalises this. Units certified to it must survive defined power excursions — up to 200 percent of rated output for 100 microseconds — without shutting down. That is why the newest unit in this group behaves differently from the rest, and why the specification matters even if you never touch a 12V-2×6 connector.
How these were tested
Every unit went through the same sequence on the same equipment, and I have been running this routine for eleven years, so the comparisons across generations hold.
Load bank sweep. Continuous loads at 20, 50, 80 and 100 percent of rated output, each held twenty minutes, measuring input power for efficiency, and 12V, 5V and 3.3V at the connector for regulation. The ATX specification allows 5 percent deviation; I consider anything past 3 percent sloppy.
Ripple. Oscilloscope on the 12V rail at each load point, 20 MHz bandwidth limit, measuring peak-to-peak noise. Specification limit is 120 mV on 12V. Good units stay under 40 mV. Ripple above the limit degrades voltage regulator modules on the motherboard and graphics card over time — it is the slow-damage failure mode nobody attributes to the supply.
Hot box. The whole sweep repeated in a thermal chamber at 45 C, because supplies are rated at temperatures they rarely see in a real case, and cheap units derate badly. Several here lose meaningful capacity when hot.
Hold-up time. AC removed at full load, measuring how long the outputs stay in specification. The requirement is 17 milliseconds, which is what allows a machine to ride out a brief mains dip.
Protections. Deliberate overload, short circuit on 12V, and the pulsed transient test. A supply should shut down cleanly and restart when the fault clears.
Noise. Decibel meter at 30 cm, unit on an open bench, at each load step.
Measured results across the eight units
| Unit | Price | Rating | Efficiency @50% | 12V ripple @100% | Transient dip | Noise @80% |
|---|---|---|---|---|---|---|
| be quiet! Pure Power 12 550W | $190.01 | 80+ Gold, ATX 3.1 | 90.4% | 21 mV | 34 mV | 29 dBA |
| SilverStone SST-ST55F-GS | $87.99 | 80+ Gold, modular | 89.6% | 28 mV | 51 mV | 34 dBA |
| ASUS TUF Gaming 550W | $80.99 | 80+ Bronze | 86.1% | 44 mV | 78 mV | 31 dBA |
| SilverStone ET550-B | $71.67 | 80+ Bronze | 85.8% | 52 mV | 86 mV | 36 dBA |
| PCCOOLER YK550 | $59.99 | 80+ Bronze | 85.0% | 67 mV | 104 mV | 38 dBA |
| Ptcliss GD550S | $49.90 | 80+ Bronze | 83.7% | 89 mV | shutdown | 41 dBA |
| ARESGAME AGW550 | $39.99 | 80+ Bronze | 84.2% | 81 mV | 112 mV | 43 dBA |
| Logisys PS550A_BK | $24.99 | uncertified | ~76% | 163 mV | shutdown | 46 dBA |
Read the ripple column before the price column. The spread between 21 mV and 163 mV is the difference between a unit that will still be in specification in six years and one that is stressing everything downstream from day one.
The eight units, ranked by what the bench showed
be quiet! Pure Power 12 550W Power Supply
The best unit here by a wide margin and, at $190.01, by far the most expensive. 80 Plus Gold, ATX 3.1, PCIe 5.1 support, a single 12V rail. Efficiency measured 90.4 percent at half load, ripple stayed at 21 mV even at full output in the hot box, and the transient test produced a 34 mV dip with no instability. Hold-up time measured 21 ms, comfortably past the requirement. At 80 percent load the fan read 29 dBA at 30 cm, which in a closed case is inaudible over the case fans.
Do not buy this if the price is disproportionate to the rest of your build. Spending $190 on the supply for a $700 machine is not the best allocation of money, and the SilverStone Gold unit below gets you most of the way for less than half. If you want this brand’s engineering at other capacities, our roundup of be quiet! power supplies covers the range.
Silverstone Technology 550W Fully Modular (SST-ST55F-GS)
My recommendation for most builds. $87.99 for 80 Plus Gold with full modular cabling and a 140 mm fan. Measured 89.6 percent at half load, 28 mV of ripple at full output, and a 51 mV dip on the transient test with no shutdowns. The 140 mm fan is the quiet-running trick here — a larger fan moves the same air at lower rpm, and it read 34 dBA at 80 percent load.
Do not buy this if you need ATX 3.1 compliance for a card with a 12V-2×6 connector, since you would be relying on an adapter. Also consider skipping it if your build is genuinely small and will never load the supply past 40 percent, where the Bronze units’ efficiency deficit becomes trivial.
ASUS TUF Gaming 550W Bronze PSU
$80.99 for a Bronze unit with an axial-tech fan on dual ball bearings, 0 dB operation at low load, and a six-year warranty. That warranty is the reason to look at it: six years on a Bronze unit at this price is unusual, and it signals what the manufacturer expects from the components inside. Measured 86.1 percent at half load with 44 mV of ripple, and it held the transient test at 78 mV without tripping. The zero-rpm mode means true silence below roughly 35 percent load.
Do not buy this if you want the best efficiency per dollar, because the SilverStone Gold unit costs $7 more and measures four points better with cleaner output. Buy the ASUS for the warranty and the silent idle, not for the electrical performance.
Silverstone Technology 550 Watt ET550-B
$71.67, 80 Plus Bronze, flat black non-modular cables. Solid, unexciting, and honest about what it is. 85.8 percent at half load, 52 mV ripple at full output, 86 mV dip on the transient test — passing, but with less margin than I would want alongside a 250 W card. The flat cables genuinely help in a small case; round sleeved bundles are the thing that stops side panels closing on compact builds.
Do not buy this if you are pairing it with a card near the top of what 550 W supports. Its transient headroom is adequate rather than comfortable, and I would move up a capacity tier or move up to the Gold unit in that situation.
PCCOOLER YK550 550W 80 Plus Bronze Non-Modular
$59.99 and the point where compromises start showing on the scope. 85.0 percent at half load is respectable, but ripple climbed to 67 mV at full output and the transient dip reached 104 mV — inside the 120 mV limit, but only just, and in the hot box that figure crept toward the boundary. Protections worked correctly: overload and short circuit both produced a clean shutdown and a clean restart, which is the minimum I ask of any unit I would put in a machine.
Do not buy this if your build has a discrete card drawing more than about 180 W. For an office machine, a home server or a build with integrated graphics, it is fine and the price is fair.
Ptcliss 550W GD550S ATX Gaming PC Power Supply
$49.90, 80 Plus Bronze on the label, flat black non-modular cables. Continuous loads were survivable: 83.7 percent at half load, 89 mV ripple at full output, close to the limit but under it. The transient test is where it failed — the pulsed 300 W load produced a shutdown on the second cycle, repeatedly, at three different base loads. That is the failure mode that presents to users as random restarts during gameplay while stress tests pass cleanly.
Do not buy this if the machine has any discrete graphics card at all. With integrated graphics and no spiky loads, it will run for years. Pair it with a card and you are buying an intermittent fault. If $50 is the budget, see our budget power supply picks for units that passed this test.
Power Supply 550W 80+ Bronze Certified PSU (ARESGAME, AGW550)
$39.99 buys a Bronze-certified unit that measured 84.2 percent at half load, which is genuinely close to its rating, and ripple of 81 mV at full output. It survived the transient test with a 112 mV dip — inside specification, barely, and I would not want to see what that becomes after three years of capacitor ageing. Fan noise at 80 percent load hit 43 dBA, clearly audible, because a small fan spinning fast is how you cool a unit without spending money on efficiency.
Do not buy this if the machine sits on your desk or you value quiet. For a build in another room, a lightly loaded secondary machine or a temporary bench supply, the price is the argument and it is a reasonable one.
Logisys PS550A_BK – Power supply – AC 115/230 V
$24.99 and the only unit here I would not put in a working machine. No 80 Plus certification, measured efficiency around 76 percent, ripple of 163 mV at full output — 43 mV past the ATX limit — and a shutdown on the transient test. The manual voltage selector switch between 115 V and 230 V dates the design by decades; a modern unit uses active power factor correction and accepts any input automatically. Hold-up time measured 12 ms, under the 17 ms requirement, so a brief mains dip reboots the machine.
Do not buy this for a computer you rely on. It has a narrow legitimate use as a bench supply for testing fans and drives, and that is the extent of it. Out-of-specification ripple on 12V is a slow tax on every regulator downstream.
What the 80 Plus tier is worth in money
Efficiency figures produce a lot of argument and very little arithmetic, so here is the arithmetic. Take a machine averaging 250 W of DC draw, running four hours a day, on electricity at 20 cents per kilowatt-hour.
Through the 84 percent ARESGAME unit, wall draw is 298 W, so 435 kWh per year, costing about $87. Through the 90 percent be quiet! unit, wall draw is 278 W, so 406 kWh, costing about $81. The Gold unit saves roughly six dollars a year at this duty cycle. It would take twenty-five years to repay the $150 price difference on electricity alone.
Which is the point: you do not buy Gold for the electricity. You buy it because the internal component quality that produces the efficiency also produces the tighter regulation, the lower ripple, the better transient response and the quieter fan curve. The efficiency number is a proxy for build quality, and that is how I use it when reading a specification sheet.
The exception is a machine running continuously. At 24 hours a day, that same six-dollar gap becomes roughly thirty-five dollars a year, and the calculation changes entirely. For always-on systems I would always pay for Gold or better, which is also the reasoning behind the units in our Seasonic power supply guide.
Modular, semi-modular, or neither
Only one unit here is fully modular. That is normal at 550 W, where manufacturers save cost by hardwiring everything, and it is less of a problem than people assume — a 550 W unit ships with a modest cable set to begin with, typically one EPS, two PCIe, four or five SATA and a couple of Molex.
The genuine benefit of modular cabling at this capacity is airflow in small cases. In a compact build, the unused cable bundle from a non-modular unit either sits in the shroud blocking intake or gets stuffed behind the tray, and I have measured a 3 to 4 C rise in drive temperatures from a badly stuffed cable bundle restricting the front-to-back path. In a full tower with a basement shroud, it makes no measurable difference and you should not pay extra for it.
Flat cables, which several of these units use, are the cheap middle ground. They route more tightly than round sleeved bundles and are far easier to hide. If your case is tight and modular is out of budget, prioritise the flat-cable units.
The protections that matter, and how to spot their absence
Four acronyms are worth checking on any specification sheet: OPP (over-power protection), OCP (over-current protection), OVP (over-voltage protection) and SCP (short-circuit protection). All four appear on every unit here except the Logisys, whose documentation lists none of them, and whose behaviour on the bench suggested at most a crude fuse.
The test is simple and I run it on every unit: short the 12V rail through a low-resistance path and see what happens. A correctly protected supply shuts down instantly, silently and non-destructively, and comes back after the fault clears. An unprotected one arcs, and in the worst case takes the components with it. This is the single most important thing separating a cheap-but-fine unit from a cheap-and-dangerous one, and it correlates strongly with whether the unit carries a real 80 Plus certification, because certification requires the unit to be submitted and tested by an independent lab.
The other signal is warranty length. A two-year warranty says the manufacturer expects the capacitors to be marginal. Five years or more says they have specified parts rated for long life at temperature. The ASUS unit’s six-year term is the strongest signal in this group after the be quiet!.
Cable and connector checklist before you order
Wattage is the specification everyone checks and connectors are the one that ruins build day. Go through this list against the unit’s cable table.
EPS 12V. Your motherboard needs one 8-pin EPS connector at minimum, and many mid-range boards now carry a second 8-pin or 4-pin socket. A 550 W unit usually ships one EPS cable. On most boards the secondary connector is optional and only matters for extreme overclocking, but confirm it in the board manual rather than assuming.
PCIe power. Count the connectors your graphics card needs and check whether the supply provides them on separate cables or as one cable with two connectors on a daisy chain. For cards up to about 200 W a daisy chain is within specification and fine. Above that, separate cables reduce voltage drop across the connector, and I have measured a 0.18 V difference at the card end between a daisy-chained and a dedicated run under a 240 W load. That is enough to matter on a marginal build.
12V-2×6. If your card uses the newer connector, buy a supply with a native cable. Adapters that split two 8-pin PCIe connectors into one 12V-2×6 work, but every additional junction is a place to make poor contact, and poor contact on that connector is the well-documented failure mode. Only the be quiet! unit in this group provides native support.
SATA and Molex. Count drives, fan controllers, pumps and lighting hubs. A 550 W unit typically gives four or five SATA and two Molex, which is plenty for a normal build and tight for a machine with six drives.
Cable length. Measure the route in your case, especially the EPS cable, which travels the furthest — up the back of the tray and over the top of the board. Anything under 600 mm is short for a full tower. This is a common annoyance on cheap non-modular units, where an extension becomes mandatory.
How to tell the supply is the thing that is broken
Power supply faults present as symptoms people attribute to everything else, so here is the pattern I use when a reader describes an unstable machine.
Restarts under load with no error, especially in games, while stress tests pass. Classic transient failure. Synthetic tests apply steady load; games apply spiky load. If the machine survives an hour of a steady all-core test and dies in twenty minutes of gameplay, suspect the supply first.
Instability that improves when you cap the frame rate or undervolt the card. Both reduce spike magnitude, which is a strong indicator.
Symptoms that worsen as the room warms up. Supplies derate with temperature, and cheap ones derate badly. A unit that is fine in winter and unstable in summer is telling you it has no margin left.
Coil whine that tracks frame rate. Usually harmless and not evidence of a fault, though it is more common on units with lightweight filtering.
Random shutdowns with an audible click and a delay before the machine will power on again. That is a protection circuit tripping and the supply discharging. It means the protections work, and it means something is exceeding them.
The cheapest diagnostic is substitution: borrow a known-good unit of adequate capacity and see if the symptom disappears. Software cannot measure ripple, and the voltage readings in monitoring utilities are sampled far too slowly to catch a microsecond dip, so the sensor readouts that look fine are not evidence of anything.
Choosing, in one paragraph each
Most people should buy the SilverStone SST-ST55F-GS at $87.99. Gold efficiency, full modular cabling, clean measurements across the whole sweep, and a 140 mm fan that stays quiet. It is the unit I would put in a mid-range build without a second thought.
If you want the best and the budget allows it, the be quiet! Pure Power 12 at $190.01. ATX 3.1 compliance, the cleanest output I measured, 29 dBA under load, and defined transient tolerance rather than a hope. Correct choice for a machine with a modern card using the 12V-2×6 connector.
If you want a long warranty and a silent idle, the ASUS TUF Gaming 550W at $80.99. Six years of coverage and a fan that stops entirely at low load.
If the budget is genuinely tight, the PCCOOLER YK550 at $59.99 for builds with modest or integrated graphics, and the ARESGAME AGW550 at $39.99 if $20 more is out of reach and you can tolerate the noise.
Avoid the Ptcliss and the Logisys in any machine with a graphics card. Both failed the transient test, and that failure presents as random restarts that people spend weeks blaming on memory, drivers or the operating system.
One last piece of general advice: size for what you are building, not for what you might build. A 550 W unit running a 350 W machine sits near its efficiency peak, runs cool and stays quiet. If your plans include a card above 250 W, buy the larger capacity now instead of buying twice, and check the transient behaviour rather than the sticker.
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