Thermaltake is a big proponent of RGB lighting. Otherwise it wouldn't have so many PSUs with built-in LEDs. The TPG-850AH3FSGR is a decent performer. However, it has to contend with formidable opponents like the Corsair RM850x and Seasonic Focus Plus Gold 850W. If you don't care about fancy lighting on your PSU, then there's no reason to prefer the TPG-850AH3FSGR over competing models that offer better performance and cost about the same (if not less).
On the other hand, if you own a windowed chassis and are into case modding, the TPG-850AH3FSGR might be an interesting option. Even if your motherboard isn't equipped to control the PSU's lighting via software, you can still dial in the settings you want through a push-button up front and count on built-in EEPROM to remember your preference.
Given a slightly lower price, the TPG-850AH3FSGR would stand up to other entries in the 850W 80 PLUS Gold-rated category. But given the extras Thermaltake integrates, it'd be hard to make this model much cheaper.
If High Power could fix the TPG-850AH3FSGR's lousy transient response, its overall performance score would climb, positively affecting the value index. Another notable downside is the lack of inrush current protection. High Power is among the few manufacturers that avoid using NTC thermistors for slightly better efficiency and lower production cost. NTC thermistors aren't particularly expensive, but bypass relays cost quite a bit more, and those are needed to isolate the thermistor once the PSU starts to avoid unnecessary energy losses. It is high time for High Power to change its strategy and use the parts needed for inrush current suppression.
We have nothing to complain about with regard to build quality, and with a bit of tuning Thermaltake could have a much more enthusiast-oriented power supply on its hands. Bottom line: if you're willing to pay for the luxury of RGB lighting, then the TPG-850AH3FSGR is a decent performer and one-of-a-kind PSU. However, if performance is a top priority, there are notably better options that cost about the same or less.
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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.