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Power Supply Unit

Power supply unit is the component of the computer that sources power from the primary source (the power coming from your wall outlet) and delivers it to its motherboard and all its components. Contrary to the common understanding, the PSU does not supply power to the computer; it instead converts the AC (Alternating Current) power from the source to the DC (Direct Current) power that the computer needs.

There are two types of PSU: Linear and Switch-mode. Linear power supplies have a built-in transformer that steps down the voltage from the main to a usable one for the individual parts of the computer. The transformer makes the Linear PSU bulky, heavy, and expensive. Modern computers have switched to the switch-mode power supply, using switches instead of a transformer for voltage regulation. They’re also more practical and economical to use because they’re smaller, lighter, and cheaper than linear power supplies.

PSU need to deliver at least the amount of power that each component requires, if it needs to deliver more, it simply won't work.

Another puzzling question for most consumers is, “Does a PSU supply constant wattage to the computer?” The answer is a flat No. The wattage you see on the PSUs casing or labels only indicates the maximum power it can supply to the system, theoretically. For example, by theory, a 500W PSU can supply a maximum of 500W to the computer. In reality, the PSU will draw a small portion of the power for itself and distributes power to each of the PC components according to its need. The amount of power the components need varies from 3.3V to 12V. If the total power of the components needs to add up to 250W, it would only use 250W of the 500W, giving you an overhead for additional components or future upgrades.

Additionally, the amount of power the PSU supplies varies during peak periods and idle times. When the components are pushed to their limits, say when a video editor maximizes the GPU for graphics-intensive tasks, it would require more power than when the computer is used for simple tasks like web-browsing. The amount of power drawn from the PSU would depend on two things; the amount of power each component requires and the tasks that each component performs.

Power supply efficiency

When PSU converts the AC power to DC, some of the power is wasted and is converted to heat. The more heat a PSU generates, the less efficient it is. Inefficient PSUs will likely damage the computer’s components or shorten their lifespans in the long run. They also draw more power from the primary source resulting in higher electricity bills for consumers.

You might’ve seen 80 PLUS stickers on PSUs or its other variants like 80 PLUS Bronze, Silver, Gold, Platinum, and Titanium. 80 PLUS is the power supply’s efficiency rating; the power supply must reach 80% efficiency to be certified. It’s a voluntary standard, which means companies don’t need to abide by the standard, but 80 PLUS certifications have become popular because a more efficient power supply can lessen the consumers’ carbon footprint and help them save some bucks on their electric bills. Below is the efficiency rating that a PSU needs to achieve to get the desired rating.

Certification Levels Efficiency at 10% Load Efficiency at 20% Load Efficiency at 50% Load Efficiency at 100% Load
80 PLUS (White) 80% 80% 80%
80 PLUS Bronze 82% 85% 82%
80 PLUS Silver 85% 88% 85%
80 PLUS Gold 87% 90% 87%
80 PLUS Platinum 90% 92% 89%
80 PLUS Titanium 90% 92% 94% 90%

It’s important to note that the 80% efficiency does not mean that the PSU will only supply 80% of its capacity to the computer. It means it will draw additional power from the primary source to only 20% of power is lost or generated as heat during the conversion. A 500W PSU will therefore draw 625W of power from the main to make it 80% efficient.

Higher efficiency also means the internal components are subjected to less heat and are likely to have a longer lifespan. They may cost a bit more, but higher certified power supplies tend to be more reliable than others. Luckily, most manufacturers offer warranties.

Power supply shopping tips

  • Determine wattage requirements: You don't need to purchase much more potential power capacity (wattage) than you’ll ever use. You can calculate roughly how much power your new or upgraded system will draw from the wall and look for a capacity point that satisfies your demands. Several power supply sellers have calculators that will give you a rough estimate of your system's power needs. You can find a few below:

  • Consider upcoming GPU power requirements: Although the best graphics cards are usually more power-efficient than previous generations, their power consumption increases overall. This is why the latest 12+4 pin connector that the upcoming generation graphics cards will use will provide up to 600 W of power. Currently, a pair of PCIe 6+2 pin connectors on dedicated cables are officially rated for up to 300W, and three of these connectors can deliver up to 450W safely. You should also add the up to 75W that the PCIe slot can provide in these numbers.

    What troubles today's power supplies is not the maximum sustained power consumption of a GPU but its power spikes, and this is why various manufacturers suggest strong PSUs for high-end graphics cards. If the PSU's over current and over power protection features are conservatively set, the PSU can shut down once the graphics card asks for increased power, even for very short periods ( nanoseconds range). This is why EVGA offers two different OPP features in its G6 and P6 units, called firmware and hardware OPP. The first triggers at lower loads, in the millisecond range, while the latter triggers at higher loads that last for some nanoseconds. This way, short power spikes from the graphics card are addressed without shutting down the system.

    If you add the increased power demands of modern high-end CPUs, you can quickly figure out why strong PSUs are necessary again. Please look at our GPU Benchmarks and CPU Benchmarks hierarchies to see how each of these chips perform relative to each other.

  • Check the physical dimensions of your case before buying: If you have a standard ATX case, whether or not it is one of the best PC cases, an ATX power supply will fit. But many higher-wattage PSUs are longer than the typical 5.5 inches. So you'll want to be sure of your cases' PSU clearance. If you have an exceptionally small or slim PC case, it may require a less typical (and more compact) SFX power supply.

  • Consider a modular power supply: If your case has lots of room behind the motherboard, or your chassis doesn't have a window or glass side, you can cable-wrap the wires you don't' need and stash them inside your rig. But if the system you're' building doesn't' have space for this, or there is no easy place to hide your cable mess, it's' worth paying extra for a modular power supply. Modular PSUs let you only plug in the power cables you need and leave the rest in the box.

Market analysis

I'm searching for a power supply unit that can deliver 264W and can grow up to 373W. This means that the load is:

Type 400W 450W 500W
Min load 66% 59% 52%
Max load 93% 83% 74%

Given that PSU look to be more efficient when they have a load of 50%, the 450W or 500W would be better. Although if the efficiency goes over 80 PLUS Gold, the difference is almost negligible. I'd prioritize first the efficiency. But any PSU from 400W to 500W will work for me.

Toms Hardware, PCGamer, IGN recommends:

  • Corsair CX450: It has 80 PLUS Bronze, so I'll discard it
  • XPG Pylon 450: It has 80 PLUS Bronze too...

None of the suggested PSU are suited for my case. I'm going to select then which is the brands that are more suggested:

Brand Tom's recommendations PCGamer recommendations IGN
Corsair 6 2 3
Be quiet 1 1 1
Silverstone 1 1 1
Cooler Master 1 0 1
XPG 1 1 0
EVGA 1 0 0
Seasonic 0 1 0

It looks that the most popular are Corsair, Be quiet and Silverstone.

It looks like I have to forget of efficiency above Gold unless I want to go with 520W. After a quick search on the provider I see that the most interesting in price are:

  • Be Quiet! Straight Power 11 450W
  • Be Quiet! Pure Power 11 CM 500W
  • Be Quiet! Pure Power 11 500W

I'm also going to trust Be Quiet on the CPU cooler so I'm happy with staying on the same brand for all fans.

Type 400W 450W 500W
Min load 66% 59% 52%
Max load 93% 83% 74%
Model Pure Power 11 CM 500W Pure Power 11 500W Straight Power 11 450W
Continuous Power 500 500 450
Peak Power 550 550 500
Min load (my case) 52% 52% 59%
Max load (my case) 74% 74% 83%
Topology Active Clamp / SR / DC/DC Active Clamp / SR / DC/DC LLC / SR / DC/DC
Fan dB(A) at 20% 9 9.3 9.4
Fan dB(A) at 50% 9.3 9.6 9.8
Fan dB(A) at 100% 18.8 21.6 12.3
dB(A) Min load 9.68 - 10.25
dB(A) Max load 13.86 - 11.45
SIP Protection Yes No Yes
Efficiency Cert Gold Gold Gold
Efficiency 20% 90.6 88.2 91
Efficiency 50% 92.1 91.3 93.1
Efficiency 100% 90.1 89.9 91.7
Cable management Semi-modular Fixed Modular
Cable cm to mb 55 55 60
Max cable length 95 95 115
No. of cables 7 7 8
ATX-MB (20+4-pin) 1 1 1
P4+4 (CPU) 1 1 1
PCI-e 6+2 (GPU) 2 2 2
SATA 6 6 8
Dimensions 160 x 150 x 86 150 x 150 x 86 160 x 150 x 86
Warranty (Years) 5 5 5
Price (EUR) 85.93 79.02 99.60

I'd discard the Pure Power 11 500W because it:

  • Has significantly worse efficiency
  • Doesn't have SIP protection
  • The fan is the loudest

Between the other two Straight Power 11 has the advantages:

  • 1% more efficiency.
  • Will make 0.5dB more noise at min load but 2.41dB less at max load. So I expect it to be more silent
  • Cables look better
  • Has more cable length
  • Has more SATA cables (equal to my number of drives)

And the disadvantages:

  • Is 13.66 EUR more expensive
  • Has 50W less of power

It doesn't look like I'm going to need the extra power, and if I need it (if I add a graphic card) then the 500W wouldn't work either. And the difference in money is not that big. Therefore I'll go with the Straight Power 11 450W