NZXT works closely with Seasonic, and this is a great advantage for this brand since it can use some of the best platforms that money can buy today. The C850 is identical to the Seasonic Focus GX-850, which is the newer version of the Seasonic SSR-850FX that we evaluated a few years ago.
There are no significant parts changes with the only exception being the primary switching FETs and the bulk cap. The C850 uses a lower capacity bulk cap, but still, it manages to offer a quite long hold-up time. The differences in load regulation, efficiency, and hold-up time didn't allow the C850 to achieve the same overall performance score as Seasonic's offerings, and it also loses to the other good choices in this category, namely the Corsair RM850x and the XPG Core Reactor 850. The significant advantage that the C850 has over the older Seasonic Focus model with similar capacity (we haven't tested yet any of the newer revisions of the Focus units) is the lower overall noise output, which is achieved thanks to the less-aggressive fan speed profile.
Although you cannot call the C850 noisy under normal operating temperatures, at least, it cannot compete in this section with the Corsair RM and RMx offerings, and it also falls behind the XPG Core Reactor 850. Given the IR images that we took under non-optimal operating conditions and the efficiency levels of this platform, there is room for a more relaxed fan speed profile. Nonetheless, the ten-year warranty that NZXT offers on this product sets the limits, and Seasonic's engineers wanted to make sure that this PSU will outlive it, even under harsh conditions. Someone could argue here, though, that the Corsair models offer the same warranty with much more relaxed fan profiles.
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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, Future PLC, are financially involved with Cybenetics. Aris does not perform the actual certifications for Cybenetics.