Years ago, SFX-based power supplies were limited to 350W or 400W, tops. Nowadays, we see 650W small form factor platforms that are not only reliable, but also relatively quiet. This is impressive because high-capacity PSUs need larger components inside for generating all of their output. It's natural, then, for SFX power supplies to have really crowded circuit boards. The problem with dense layouts is that they limit airflow quite a bit. Moreover, it's hard to find enough room for large heat sinks. As a result, building small, powerful PSUs with conservative fan profiles is immensely difficult. The best way to achieve this is minimizing waste heat with higher efficiency.
Because they're so small, it's unfair to compare SFX-based PSUs to standard ATX models. After all, large PSUs aren't constrained by as many physical variables, enabling better performance and quieter operation. Moreover, PSUs with small footprints tend to cost more. That's exactly the case with SilverStone's SX650-G, which sells for about $135 on Newegg. The SX650-G does perform well for an SFX power supply, and if it wasn't for the superb Corsair SF600 and SF600 Platinum, SilverStone's model would play alone in this segment.
However the SF600 sells for less money than the SX650-G and its overall performance is much higher. The SF600 might be louder in our cross-load tests, but it's a special case. Normally, load on the minor rails doesn't have a significant effect on fan speeds. In the case of Corsair's PSU, though, high loads on the 5V and 3.3V rails make the fan spin up. Fortunately, under normal usage, load on the minor rails is low.
The SX650-G can handle elevated operating temperatures, it is especially powerful for an SFX-based PSU, it boasts a fully modular design, it's quiet under light and moderate loads, and it fares well in our performance benchmarks. We would like to see SilverStone expose a second EPS connector and implement a smoother fan profile, since the existing one transitions between low and high speeds roughly.
Compared to the slightly cheaper Corsair SF600, SilverStone's SX650-G officially offers a 50W-higher capacity and two additional PCIe connectors. If you're building a PC with multiple graphics cards, that's a big deal. On the other hand, Corsair's SF600 serves up a much longer warranty period. Moreover, its overall performance is notably better. SilverStone needs a lower price tag to give the SX650-G a clear advantage. Otherwise, it's one of the few SFX models featuring 650W of capacity and four PCIe connectors.
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Disclaimer: Aris Mpitziopoulos is Tom's Hardware's PSU reviewer. He is also the Chief Testing Engineer of Cybenetics, and developed the Cybenetics certification methodologies apart from his role on Tom's Hardware. Neither Tom's Hardware nor its parent company, Purch Media, are financially involved with Cybenetics. Aris does not perform the actual certifications for Cybenetics.