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Corsair CS850M 850W Power Supply Review

Efficiency, Temperatures And Noise


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

Under normal loads, the unit's efficiency lands it in the middle of its market segment. At light loads, the CS850M scores a second-place finish behind the high-end Cooler Master V850, which utilizes a cutting-edge platform that costs significantly more. In general, Corsair's offering performs well in our efficiency tests. That should make prospective customers happy, since they'll save money on power over time.

Efficiency at Low Loads

In the next tests, we measure the efficiency of the CS850M at loads significantly lower than 10 percent of the device's 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 #12V5V3.3V5 VSB PowerDC/AC (W)Efficiency (%)Fan Speed (RPM)Fan Noise (dB[A])PF/AC (V)

Efficiency is pretty good at low loads. As you can see in the above table, even under a mere 20W load, the CS850M stays above 65 percent. Also, the fan spins at low speeds during these tests, despite the greater-than 35 °C temperature inside our hot box. 

5VSB Efficiency

The ATX specification states that 5VSB standby supply efficiency should be as high as possible, recommending 50 percent or higher efficiency with 100mA of load, 60 percent or higher with 250mA of load and 70 percent or higher with 1A or more of load.

We will take four measurements at 100, 250 and 1000mA, and one under the highest load the 5VSB rail can handle. 

Test #5VSBPower DC/AC (W)Efficiency (%)PF/AC (V)

The 5VSB rail's efficiency is among the best we have seen in the tests with 0.25 and 1A loads. The only thing spoiling the overall picture is a significant efficiency drop during the full load test, which indicates that 3A on this rail pushes the 5VSB regulation circuit hard.

Power Consumption in Idle And Standby

Mode12V5V3.3V5VSBPower (AC)PF/AC Volts

In the table above, 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), along with the power consumption when the PSU is in standby mode (without any load at 5VSB).

With 115VAC input, the PSU’s energy demands are very low. They increase with 230VAC, though the CS850M still meets the strict ErP Lot 6 2013 requirements.

Fan RPM, Delta Temperature And Output Noise

The above chart illustrates the cooling fan's speed (in RPM), along with the delta between input and output temperature. The results were obtained at 35 to 48 °C ambient temperature.   

This chart shows the cooling fan's speed (in RPM) and output noise. We measured acoustics from one meter away, inside a small, custom-made anechoic chamber with internals completely covered in soundproofing material (be quiet! Noise Absorber kit). Background noise inside the anechoic chamber was below 18 dB(A) during testing, and the results were obtained with the PSU operating at 35 to 48 °C ambient temperature. 

The following graph illustrates the fan's output noise over the PSU's entire operating range. The same conditions of the above graph apply to our measurements, though the ambient temperature was between 28 and 30 °C.  

At up to around 450W, the CS850M is pretty quiet; it starts to make its presence known with more than 550W of load. In general, this PSU isn't obtrusive, though it doesn't benefit from a semi-passive mode under light loads.

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