Is a Higher Wattage RM PSU Quieter Than a Smaller One?

A few weeks ago, we presented the question: "does a bigger PSU mean a quieter PSU"? Using an AX760i and an AX1200i in the same PC, we found that the AX1200i was significantly quieter when presented with a 600W load. Of course, the AXi product line is the absolute top of the line and the differences between the AX760i and the AX1200i go far beyond how the fan controller is programmed. So this week we're going to do the same thing, but with a different set of power supplies.

Because we recycle systems at Corsair quite often, the build is a little different:

Motherboard: ASUS Z97-WS
CPU: I7-4770K @ 4.3GHz 1.25Vcore
CPU Cooler: Corsair H100i with stock 2x 120mm fans
RAM:16GB (4x4GB) Corsair Dominator Platinum DDR3-2800 CAS12
Graphics card:AMD Radeon HD 7990 6GB GDDR5 (Reference)
OS drive:120GB Corsair Force GT SSD
Drive:240GB Corsair Force LS SSD
OS:Windows 7 SP1 x64
Chassis:Obsidian 450D Case w/ stock 2x 140mm intake fans and 1x 120mm exhaust fan

Room temperature has been measured as low as 24.7°C and as high as 25.8°C.

To make sure we're fully loaded, we're running Prime95 (two threads) and the  Unigine Valley benchmark simultaneously.

Because the RM series power supplies are not digital, the monitoring capabilities are limited compared to that of the AXi series. By using an analog to digital bridge , we can monitor the +12V load on the PSU and the fan RPM. We'll log the CPU temperature to illustrate the PC's activity.

You can click on any of these graphs to get a better view.

We'll start with the RM550....

If we look at the "12V current" line in blue, we can see that we peak out at almost 37A. That equates to around 444W, so we're pretty close to fully loading this power supply.

Now, we can't measure the PSU temperature with the RM Series, but we're still going to log the CPU temperature so we have an idea of how hot things are. The CPU gets up to 82.1°C. Using a K-Type thermal probe taped to the housing of the PSU, we find that the ambient temperature by the PSU is around 33.5°C.

The fan almost immediately starts up and fluctuates between 250 and 650 RPM before settling in on an average fan speed of 650 RPM.

Now we'll swap out the 550W for the RM650 and wait about an hour for the ambient temperatures to drop to under 24°C.

You can see that our +12V load is pretty much the same, as it should be as we're running all of the same benchmarks on the same hardware, and we see the fan once again fluctuating between 250 and 650 RPM.

This time, however, the fan goes up to 800 RPM once the inside of the PC reaches over 30°C according to our K-type thermometer. We hit temperatures inside the PC as high as 30.7°C. which is a good 2.8°C lower than the first test, despite the fact that room temperature is only 1.2°C cooler at the most. The CPU temperature is slightly lower too, which leads us to believe that the higher fan RPM is reducing temperatures inside the case and helping reduce CPU temperatures.

Now let's try the RM750. While the RM550 and 650 are based off of the same platform, the RM750 is a completely different animal internally.

And here we can see that with a completely different platform comes completely different results.

The fan stays off longer, at first, and for the next six minutes, the fan switches from off to spinning 690 RPM. Finally, the fan settles on an average RPM of around 680 RPM. So while initially the RM750 is quieter, it only takes a matter of time before it's the same as the 550W.

But what about the RM850? It's based on pretty much the same platform as the RM750...

The RM850 doesn't seem to be too different than the RM750 in terms of fan speed. Once again, we see the PSU trying to hang onto Zero-RPM mode for a much longer duration than the RM550 or RM650, but it eventually does kick in and spin at 650 RPM average. Only slightly slower than the 690 RPM we logged with the RM750.

Finally, we're going to install an RM1000 in this PC. The RM1000 is a different platform still. Different than the RM450 and RM550 and different from the RM750 and RM850 as well. Let's see how it does!

A typical user would assume that their fan isn't working at all! After more than 20 minutes, the outer surface of the PSU housing reaches 40.9°C, the hottest we've seen, and this causes the fan to immediately spin up to 900 RPM. Of course, the fan only stays up here for less than a minute before spinning back down and leveling off at 620 RPM where it stays for almost two minutes before shutting off again.

Not only was the RM1000 the quietest of the bunch, we also found it was the most efficient. We could only log the +12V load in Link with the RM series power supplies nd not total DC output. And we certainly aren't able to log AC input and calculate and log efficiency on the fly like we can with the AXi series. But we used a Watts-Up meter to measure and log the AC load at the wall. The RM450 and RM550 consumed 547W from the wall. The RM750 and RM850 consumed a little less at 541W. The RM1000 consumed the least amount of power at 534W. Of course, this makes sense as a power supply tends to be the most efficient at 50% of its capability and less so at lower and higher loads.

Once again, we've shown that a larger PSU can potentially be a quieter unit, though the actual difference can be minimal when looking at models that are only a few wattages apart and certainly within the same platform. More importantly, PSU choice should be purpose driven. If you know your PC will be used primarily for office applications and typically used around the neighborhood of 100 to 150W, it doesn't make sense to buy a 1000W, 850W or even a 750W if you're only going to be occasionally gaming. Even if you have a killer graphics card installed just for the occasional game and the PSU is 3% more efficient at 50% load than at 10% or 20% load, you're not going to see much of an ROI buying a larger PSU if less than 10% of your computer's usage is gaming. And it's likely the low noise at high load benefit won't be that noticeable as the graphics card fan is typically A LOT louder than any PSU fan. On the other hand, if you're someone that has a PC strictly for gaming and 90% of the time that PC is in game, it makes sense to get a PSU that's more than you need so it can run more efficiently, quieter and allow for plenty of overhead if you plan to either add an additional graphics card or replace your existing one with a more powerful card some time down the road.