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Corsair RM850x White PSU Review

Efficiency, Temperature & Noise


Our efficiency testing procedure is detailed here.

Using results from the previous page, we plotted a chart showing the RM850x's efficiency at low loads, and loads from 10 to 110 percent of its maximum-rated capacity.

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Compared to similar-capacity units with the same 80 PLUS rating, Corsair's RM850x fares well under normal loads. Under light loads, it doesn't land far from its competition. However, it does land in last place.

Efficiency At Low Loads

In the following tests, we measure the RM850x's efficiency at loads significantly lower than 10 percent of its maximum capacity (the lowest load the 80 PLUS standard measures). The loads we dialed were 20, 40, 60, and 80W. This is important for representing when a PC is idle, with power-saving features turned on.

Test #12V5V3.3V5VSBDC/AC (Watts)EfficiencyFan SpeedPSU NoisePF/AC Volts
11.166A0.492A0.477A0.199A19.21467.377%0 RPM<6.0 dB(A)0.824
22.418A0.988A0.989A0.397A39.59280.151%0 RPM<6.0 dB(A)0.930
33.608A1.485A1.470A5.028A59.12383.758%0 RPM<6.0 dB(A)0.954
44.863A1.980A1.982A0.796A79.52186.122%0 RPM<6.0 dB(A)0.965

We would like to see over 70% efficiency in the 20W load test, but the RM850x doesn't pull this off. In the other three tests, though, efficiency exceeds the 80% mark.

In each test, the RM850x operates in passive mode. Noise output is naturally negligible.

5VSB Efficiency

The ATX specification, along with CEC, ErP Lot 3 2014 and ErP Lot 6 2010/2013, states that 5VSB standby supply efficiency should be as high as possible, recommending 75 percent or higher with 550mA, 1A, and 1.5A of load. The PSU should also achieve higher than 75% efficiency at 5VSB under full load, or with 3A if its max current output on this rail is higher than 3A.

We take six measurements: one each at 100, 250, 550, 1000, and 1500mA, and one with the full load the 5VSB rail can handle.

Test #5VSBDC/AC (Watts)EfficiencyPF/AC Volts
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Besides great load regulation, the 5VSB rail is also highly efficient. Once more, CWT shows that it builds high-performance 5VSB circuits into its platforms.

Power Consumption In Idle And Standby

In the table below, you'll find the power consumption and voltage values of all rails (except -12V) when the PSU is idle (powered on, but without any load on its rails), and the power consumption when the PSU is in standby mode (without any load, at 5VSB).

Mode12V5V3.3V5VSBWattsPF/AC Volts
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Power consumption in standby mode is pretty low with both voltage inputs (115V and 230V).

Fan RPM, Delta Temperature, And Output Noise

Our mixed noise testing is described in detail here.

The first chart below illustrates the cooling fan's speed (in RPM), and the delta between input and output temperature. The results were obtained at 37°C (98.6°F) to 47°C (116.6°F) ambient temperature.   

The next chart shows the cooling fan's speed (again, in RPM) and output noise. We measured acoustics from one meter away, inside a hemi-anechoic chamber. Background noise inside the chamber was below 6 dB(A) during testing (it's actually much lower, but our sound meter’s microphone hits its floor), and the results were obtained with the PSU operating at 37°C (98.6°F) to 47°C (116.6°F) ambient temperature. 

The following graph illustrates the fan's output noise over the PSU's operating range. The same conditions of the above graph apply to our measurements, though the ambient temperature was between 30°C (86°F) to 32°C (89.6°F).  

Corsair's passive mode doesn't last long in our cross-load tests. In fact, it's totally overlapped by the next level/color (6-10 dB[A]/black). Somewhere in the center of the black bars, the PSU runs without its fan spinning, but just briefly. We have seen this before from CWT platforms. Once you push the minor rails hard, the cooling fan starts spinning. This might be a glitch, or CWT simply believes it's important to cool the minor rails well once their FETs start working overtime.

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Aris Mpitziopoulos
Aris Mpitziopoulos is a Contributing Editor at Tom's Hardware US, covering PSUs.