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Corsair RM850x (2018) PSU Review


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.

With normal loads the RM850x takes over the second place while with light loads it falls a little behind.

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 dial are 20, 40, 60, and 80W. This is important for representing when a PC is idle, with power-saving features turned on.

Test # 12V 5V 3.3V 5VSB DC/AC (Watts) Efficiency Fan Speed (RPM) PSU Noise (dB[A]) PF/AC Volts
1 1.194A 0.495A 0.482A 0.198A 19.562 67.048% 0 <6.0 0.874
12.108V 5.053V 3.328V 5.048V 29.176 115.05V
2 2.451A 0.989A 0.991A 0.397A 39.984 78.325% 0 <6.0 0.936
12.112V 5.053V 3.327V 5.044V 51.049 115.05V
3 3.638A 1.484A 1.474A 5.041A 59.472 82.943% 0 <6.0 0.961
12.114V 5.053V 3.327V 5.041V 71.702 115.05V
4 4.897A 1.979A 1.986A 0.794A 79.922 86.304% 0 <6.0 0.966
12.113V 5.053V 3.326V 5.037V 92.605 115.05V

It would be ideal if the registered efficiency with 20W load was closer or over 70%, while with 40W we would like to see above 80%. In all four light-load tests the PSU’s fan is not engaged. It needs much higher loads (and temperatures), for the fan to start spinning.

5VSB Efficiency

The ATX specification (revision 1.4), along with CEC, ErP Lot 3 2014 and ErP Lot 6 2010/2013, states that the 5VSB standby supply efficiency should be as high as possible, recommending 75 percent or higher with 550mA, 1A, and 1.5A of load.

The supply 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 # 5VSB DC/AC
Efficiency PF/AC Volts
1 0.100A 0.505 75.940% 0.062
5.047V 0.665 115.05V
2 0.250A 1.261 78.080% 0.141
5.043V 1.615 115.05V
3 0.550A 2.769 78.822% 0.251
5.033V 3.513 115.06V
4 1.000A 5.021 78.124% 0.340
5.020V 6.427 115.06V
5 1.500A 7.510 77.775% 0.391
5.006V 9.656 115.06V
6 3.000A 14.897 76.258% 0.453
4.965V 19.535 115.06V

The 5VSB rail’s efficiency is close to the competition but still on the low side. It would be great to see at least one reading close to 80%.

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 unit is in standby mode (without any load, at 5VSB).

Mode 12V 5V 3.3V 5VSB Watts PF/AC Volts
Idle 12.101V 5.048V 3.326V 5.047V 7.912 0.518
Standby 0.032 0.003


The vampire power levels are very low, with both 115V and 230V input.

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 measure acoustics from one meter away, inside a hemi-anechoic chamber. Background noise inside the chamber is below 6 dB(A) during testing (it’s actually much lower, but our sound meter’s microphone hits its floor), and the results are 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 is between 30°C (86°F) to 32°C (89.6°F).

Once you start pushing the minor rails the semi-passive operation ends prematurely. Nevertheless, the fan spins at very low speeds keeping the overall noise output low. Under normal operating temperatures the PSU’s noise won’t exceed 30 dB(A), even with prolonged full load operation.

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