The Math Behind GPU Power Consumption And PSUs

Measuring Power Consumption: A Practical Implementation

What Goes On At The Motherboard Slot?

Let’s start small and modest, since not all graphics cards have additional power connectors. In fact, there was a time when they didn’t need any. Whether or not they do depends on power consumption. A graphics card can draw up to a maximum of 75W through its PCI Express slot.

We’re looking at several voltage rails here: 3.3, 5 and 12V. As mentioned, these voltages are looped through the motherboard and can thus be measured at the 24-pin power connector, since the motherboard doesn’t influence them.

Once again, I measure the currents present on the motherboard's 3.3 and 12V rails, between the motherboard and graphics card, using the riser card's two external loops.

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Load Spikes On The Motherboard: Are They Dangerous?

I’ve measured different approaches to graphics card power supply used by various manufacturers over the past few months. I started with smaller cards, such as variations on Nvidia's GeForce GTX 750 Ti. They don’t really draw more than 60W on average, but they can produce large peaks depending on the specific model and clock rate. This can result in them breaking through the 75W barrier quite noticeably for fractions of a second, witnessed in the graph below.

For some of these mainstream boards, I’ve measured up to 120W in places, which is way higher than it should be according to the specifications! Since they're short spikes, the motherboard doesn't get damaged. This is because the connectors lead straight to the 24-pin ATX lead, which connects the motherboard to the PSU with at least two 12V supply lines.

What could cause real problems though, especially in conjunction with other components on the motherboard, is that the peaks happen at high frequencies. On inexpensive motherboards, the graphics card can actually be “heard” via on-board sound. Maybe you've already observed a popping noise while scrolling; now you know what causes it.

That's not a crappy on-board codec or a defective motherboard. Rather, the noise is caused by the graphics card’s spikes humming along.

Depending on the model, even high-end cards draw power through their PCI Express slot, in spite of their auxiliary cables. And that's why those boards have two separate voltage converter areas. This also explains how the load distribution between 12V rails changes depending on whether the graphics card is idle or under load. Other models, such as AMD's Radeon R9 295X2, don’t really use the motherboard’s power supply at all any more.

Sustained Power And A Roller Coaster Ride At The PCIe Connector

For our grand finale, we’ll look at one of those cards that barely uses the motherboard’s PCIe slot for power. This card peaks at more than 400W one moment, just to fall all the way to 40W the next.

Our measurement is taken at each auxiliary power connector. Of course, this is unavoidable for graphics cards like the AMD Radeon R9 295X2, since each connector supplies power to one of the two GPUs.

But let’s get back to the water-cooled Radeon R9 290X in our diagrams. The card's TDP is set at 250W, which it doesn’t quite reach, even during stress testing. To round out our observations, there's a table showing the ups and downs for gaming, below. It demonstrates nicely just how different loads on the different PCIe connectors can be.

Amazing, isn’t it? Then again, there are much more extreme cases, which we get to on the next page.

HIS R9 290X IceQ

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