be quiet! SFX Power 2 300W Power Supply Review

by - 6 years ago


Introduction & Packaging


When a compact ATX power supply just isn’t compact enough you have to think outside the box, or use a new form factor. The SFX and TFX form factors are two of the most common small form factor power supplies aside from the standard ATX that is so common in desktops. The SFX form factor measures in at 100×125×63.5 mm compared to TFX at 146×83×64mm. SFX is short and stubby while TFX is long and thin. In that sense TFX is more server orientated when server racks tend to be long and thin. Conversely SFX is more orientated towards compact computer systems that are not so obscurely shaped. Very few power supply vendors produce SFX or TFX units for the consumer market due to the fact demand is so low. However, be quiet! being the smart company they are have seen the trend towards SFX and TFX power supplies in recent years as small form factor builds have become more fashionable.

Today we are looking at the be quiet! SFX Power 2 300W power supply which conforms to that minuscule SFX standard. 300 watts may not sound like a lot but with an Intel Core i7 4770K and GTX 750 Ti based system you could run a complete gaming rig off of 100 watts or so. With an extra 200 watts to spare there is scope for anything up to a GTX 770 in theory, providing you don’t overclock either too hard. Lian Li, Thermaltake, SilverStone and many more case vendors produce compact cases taking advantage of SFX PSUs. Therefore what I am getting at is the fact that building a tiny SFX based system has never been easier to do.


be quiet! offer 300 and 400W SFX power supply units: that 400W model gives you a lot of scope to go for a proper high end system. Sadly when we look at the cable options we see that you are limited by the single PCIe 6+2 pin connector. Determined system builders could make do with funneling the 12 volt amps through molex or SATA to PCIe adapters, the quality of the platform should allow this to be viable. However, the main issue will be whether the 12 volt rail is capable of delivering enough amps for a high end graphics card. My recommendation would be no higher than an R9 270 or GTX 750 Ti but you might be able to squeeze out more.


Packaging and Contents

Small power supply, small box. The SFX Power 2 comes with compact packaging and be quiet! are claiming compact and silent energy. Somehow I don’t think this will be the case; generally small fans on small power supplies end up being noisy.

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The included accessories are standard for most be quiet! products: a power cable, user manual, set of cable ties and black screws.

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A Closer Look – Exterior

Power Supply Unit

The SFX form factor is very compact as we’ve already stated and so an 80mm fan is the choice for cooling.

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Both sides are plain with one side having some holes for additional ventilation.

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On the bottom is the sticker with power details and certifications. be quiet! warn customers about removing the cover – but we went ahead and did it anyway on the next page! (and I am still alive to tell the tale…)

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At the cable end we find a little more ventilation, all of the fixed cables and a screw holding the cover to an internal heatsink for heat dissipation.

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At the rear is a power switch, some hexagonal mesh and a kettle lead input which supports 100 to 240v input ranges.

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The cabling is fairly modest as this is only a 300W unit. We’ve got a 24 pin, one PCIe 6+2 pin, a CPU 4 pin, four SATA, one molex and a FDD.

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A Closer Look – Interior

Internally the OEM for this unit is very likely to be FSP since be quiet! rarely work with anyone else.

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The primary side is provided by a CapXcon 180uF, 420V and 105 degrees celsius rated hold-up capacitor.

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The smaller filtering capacitors for the secondary side are of similar quality standards and are rated for high temperature operation.

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The level of AC line filtering for this unit is also impressive, just look how they managed to squeeze all of the X & Y capacitors, MOVs and CM chokes into such a neat arrangement.

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The 80mm fan is made by Protechnic Electric and has the part number MGA8012YS-A15. No details of its maximum rated RPM can be found but given the 80mm form factor and the 0.28 amps I think 3000-3500 RPM is likely.

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Test Procedure

At eTeknix we take the power supply testing procedure very seriously and have invested a lot of resources into acquiring the appropriate testing equipment. For all power supply reviews we test the power supplies with dedicated power supply testing equipment. This means we are able to get the most accurate results from our testing as opposed to using software benchmarks (such as OCCT) or multi-meter readouts which are broadly inaccurate.


Our test machinery is as follows:

  • Sunmoon SM-5500ATE Active Load Tester (1200W rated)
  • Stingray DS1M12 USB Oscilloscope
  • Voltcraft DT-10L laser tachometer

The eTeknix test procedure involves:

  • Testing each power supply at 20/40/60/80/100% load (with balanced load across all rails) and measuring PFC (power factor correction), efficiency (actual power divided by power “pulled at the wall”) and voltage regulation (deviance from expected voltages of 3.3/5/12 on the main rails).
  • Measuring ripple with an oscilloscope at 20/40/60/80/100% load.
  • Measuring fan speed after a stabilisation period of five minutes at each load scenario using the Voltcraft DT-10L laser tachometer and a reflective strip on the fan.
  • Testing each power supply’s OPP (Over Power Protection) mechanism and seeing how many watts each power supply can deliver before shutting down

Other things to consider are that

  • We recognise that a single yellow 12 volt cable can provide only 6 Amps before overheating (which corrupts voltage regulation and efficiency) and so we used an adequate number of cables for each power supply to ensure there is not efficiency loss from poor cables selection
  • Our Sunmoon SM-5500ATE power supply tester is not capable of testing more than 300W on each of the 12 volt rails so where a power supply provides more than 300W on a 12 volt rail that power is distributed over multiple 12 volt rails on the load tester. For example a power supply with one 12 volt rail supplying 750 watts would be spread equally over three 12 volt rails on the load tester, a power supply with two 450W 12v rails would be spread over four 12v rails on the load tester, two 225W 12v rails for each of the 12v rails on the unit.
  • We use the same time scale and horizontal millivolt scale on our oscilloscope for all ripple tests, that is a 20ms T/DIV (horizontal) and a 0.02 V/DIV (vertical) meaning the scale is from -80mV to +80mV, ATX spec dictates that the 12v rail must fall within 150mv of ripple and the 3.3/5 within 50mv so that scale allows us to include both 150 and 50mV peaks. (Some older PSU reviews use different scales which were later ditched as the visual representation they give is inadequate, in these reviews written measurements are provided only).
  • Deviance is the terminology used to represent the way voltages diverge from the expected values

Efficiency, PFC and Voltage Regulation

Voltage Regulation

To test voltage regulation we load the power supply to five different load scenarios that give an equal spread of load across every single rail. So that means 20% on all rails, 40% on all rails and so on. We then calculate the average deviance of each rail from its expected voltage.


Voltage regulation was impressively strong across all rails for such a small unit, only the irrelevant minus 12 volt rail showed weak performance. The minus 12 volt rail is practically irrelevant because the vast majority of modern PCs no longer use this rail.

Power Efficiency

Power efficiency is measured by calculating actual supplied wattage divided by the wattage drawn at the wall/plug, multiplied by 100 to give a percentage. We then compare that to the particular 80 Plus certification the company claims to see if it meets that. You can see the 80 Plus certifications below, we always test 230v power supplies.



Efficiency was met and exceeded the 80 Plus Bronze baseline by quite some margin, it wasn’t, however, high enough to reach the next certification up: 80 Plus Silver.

Power Factor Correction

Power Factor Correction is the ratio of the real power flowing to the load, to the apparent power in the circuit. The aim of PFC is to make the load circuitry that is power factor corrected appear purely resistive (apparent power equal to real power). In this case, the voltage and current are in phase and the reactive power consumption is zero. The closer the number to one the better as this allows the most efficient delivery of electrical power (Source – Wikipedia).


PFC took a hit at lower loads, most power supplies normally ramp up to 0.95+ from 40% load with only 20% load having the lower readings.



Noise and Ripple can easily be measured by an oscilloscope. These show how much voltage fluctuation there is on a particular rail. We tested the rail stability of the 3.3 volt, 5 volt and 12 volt rails using an identical time and millivolt scale for all graphs. millivolt ripple is measured by the peak to peak size of the voltage curve.


Sample Ripple Graph

The latest ATX 12 volt version 2.3 specifications state that ripple from peak to peak must be no higher than 50 millivolts for the 3.3 volt and 5 volt rails, while the 12 volt rail is allowed up to 120 millivolts peak to peak to stay within specifications. Millivolt figures are stated to the closest increment of 5 given their variability.

Load (%) 3.3V Ripple 5V Ripple 12V Ripple
20  7mV  5.2mV  14.6mV
40  8.6mV  6.6mV  18.4mV
60  11mV  8.8mV  23.2mV
80  13.8mV  12.2mV  31.2mV
100  16.6mV  12.8mV  35.8mV

Ripple suppression was excellent for such a compact unit; less than 40 mV on the 12 volt and around 15mV on the other main rails is an excellent result. We see results similar to this on higher end ATX units.

3.3 volt @ 100%

5 volt @ 100%


12 volt @ 100%



Over Power Protection and Max Wattage

Power supplies often quote as having various protection mechanisms such and the most important of these is Over Power Protection. In our testing we crank up the power draw until the power supply either shuts down (meaning the OPP mechanism is present and working) or blows up (meaning it is either not present or not working). We then note the maximum power consumption before the power supply shut down (or blew up).


OPP kicked in just as expected, we were able to grab another 85W before this unit shut down – not a bad showing.


Fan Speed

When testing in a power supply laboratory it is difficult to take fan noise readings as the noise from the Sunmoon test equipment and air conditioning corrupts everything. The next best thing in our circumstances was reading off the fan speed with a tachometer to get an idea for the noise. The ambient temperature during testing held constant at 22 degrees, with 1 degree of variation. Each power supply had a consistent time period of 5 minutes to stabilise between each load scenario. 

In my experience the following general relationships apply between noise levels and fan speeds, though it can vary greatly between the type of fan used.

  • Below 800 RPM – Inaudible/Silent
  • 800 to 1000 RPM – Barely audible
  • 1000 – 1200 RPM – Audible but still quiet
  • 1200 – 1400 RPM – Moderately noisy
  • 1400 – 1800 RPM – Noisy
  • 1800 RPM or higher – Intolerable


The above fan guideline is mainly for the usual 120,135 and 140mm fans equipped on power supplies. For 80mm fans it is very different. At 1300 RPM an 80mm fan is near inaudible, after about 2000RPM it does become noisy and nearing 3000 RPM it was very noisy. Overall the be quiet! SFX power 2 is quiet for 0-60% operation and after that the noise kicks up. To be fair this is a limitation of the form factor, it is difficult to cool 300 watts of DC power conversion and regulation in such a small space.


Final Thoughts


be quiet!’s SFX Power 2 300W power supply is available in the UK at Scan Computers for £41, it comes with a 2 year warranty. We could not find the product for sale in the USA.


If you’re in the market right now for an SFX form factor power supply your options are very limited to a handful of brands. Despite these limitations be quiet! have not been complacent at all: their SFX Power 2 300W is a high quality power supply unit that delivers excellent performance for its size. Voltage regulation is strong, efficiency is better than certified and the unit provides quiet operation up to about 60% load. be quiet! have also equipped a modest but wattage-sensible number of cables and connectors. Furthermore, considering the limited availability of SFX power supplies in general the price is pretty good too.

A few things could do with improvement though. While PFC at 60-100% load was good, it needs improving at lower loads. I also felt that the fan profile was a bit steep with it not changing much until 80% load where it made a big jump in RPM, this makes the unit very noisy at higher loads. Minus 12 volt regulation was a little sloppy too but as we’ve said the minus 12 volt rail is insignificant and I’m not even sure why modern PSUs continue to equip it.


  • Competitive pricing
  • Compact size – ideal for SFF builds
  • Strong efficiency, voltage regulation and ripple supression


  • Sudden fan profile – noisy at 80/100% load but quiet below that
  • PFC at low loads not the best

“When small just isn’t small enough be quiet!’s SFX Power 2 300W power supply offers a great solution to the space problem. If you’re building an ultra compact system you can be rest assured the SFX Power 2 300W will still provide you with high quality power, just in a smaller package.”

eTeknix Innovation Award

be quiet! SFX Power 2 300W Non-Modular Power Supply

Thank you to be quiet! for providing this review sample.

Article Index

  1. Introduction and Packaging
  2. A Closer Look - Exterior
  3. A Closer Look - Interior
  4. Test Procedure
  5. Efficiency, PFC and Voltage Regulation
  6. Ripple Testing
  7. OPP and Max Wattage
  8. Fan Speed
  9. Final Thoughts
  10. View All

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