Intel’s Misleading Thermal Design Power (TDP): Don’t Trust Your Processor’s Wattage Rating

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Image credit: ewfewfev.blogspot.com

Intel has come under scrutiny lately for the power consumption and heat output of its CPUs, specifically the 9th Generation Core lineup consisting of Coffee Lake and Skylake-X parts. This much is understandable, as a revitalized AMD has forced Intel to increase core counts and clock their chips aggressively. However, the rating Intel gives its chips that’s supposed to inform users how much heat it outputs — the Thermal Design Power (TDP) — has not risen substantially in turn. Let’s just take a look at a cross-section of processors:

CPU Intel Core i7-7700K Intel Core i7-8700K Intel Core i9-9900K Intel Core i7-6950X Intel Core i9-7980XE Intel Core i9-9980XE
Node 14nm+ 14nm++ 14nm++ 14nm 14nm+ 14nm++
uArch Kaby Lake (Skylake) Coffee Lake (Skylake) Coffee Lake (Skylake) Broadwell Skylake-X Skylake-X
Cores / Threads
4 / 8 6 / 12 8 / 16 10 / 20 18 / 36 18 / 36
All-Core Turbo
4.4GHz 4.3GHz 4.7GHz 3.4GHz 3.4GHz 3.8GHz
TDP 91W 95W 95W 140W 165W 165W

The Core i9-9900K, despite being practically double the CPU and running at a higher clock speed on the same manufacturing node, somehow is only rated for an extra 4W of heat. This heat output rating is closely correlated, but not entirely equivalent to, the chip’s power consumption, as the vast majority of power drawn by the chip is dissipated as heat.

The secret to the formula is the CPU’s base clock, the lower advertised speed. In the footnotes, Intel’s TDP rating is technically only valid for the base clock. This is generally considered as a bottom floor for the CPU’s frequency under normal load scenarios. When idling, the chips run at a far lower frequency, and when loaded as long as there’s no thermal throttling (or AVX-512 instructions) they should run at a higher frequency. The i7-7700K has a base clock of 4.2GHz, compared to 3.6GHz for the 9900K. The 7700K only goes 200MHz above the speed for which it’s rated 91W, whereas the 9900K climbs a massive 1.1GHz.

Another element is the fact that TDP is generally an imprecise metric. Historically, since the initial generation of Core processors, Intel’s TDP has always been very liberal. The TDP rating was an absolute worst-case scenario; your chip was likely to consume dozens of watts less. The 7700K tends to consume up to 10W less than its TDP would indicate despite running at 4.4GHz on all cores instead of 4.2GHz.

Now, as you might have guessed, reviewers have generally found the Core i9-9900K to consume somewhere within the range of 150W and 180W of power under load with the default BIOS configuration. It’s possible to limit the CPU to its TDP in the BIOS, as is the case by default with many pre-built systems, but this results in lower sustained clock speeds and noticeably worse performance than reviews would indicate.

Why is an accurate TDP rating important? For one, it helps a user decide how beefy a power supply they need to run the chip. Secondly, it’s supposed to inform users — and OEMs like Dell and HP — how beefy a cooler is required to keep the chip at acceptable temperatures. In fact, Intel’s own decisions about which cooler to include in the box are based on TDP (the 9900K and 9980XE don’t come with coolers, hint hint). The Core i7-8700 has a TDP of just 65W and comes with a cooler rated for 73W for good measure. However, it actually has the same all-core turbo as the i7-8700K, despite a 500MHz lower base clock. If you try to run it with the stock cooler in a consumer motherboard, it will immediately overheat and throttle under load, as Tom’s Hardware has demonstrated.

AMD’s TDP for their Ryzen parts, on the other hand, is by all indications very accurate — which they ought to be commended for, even though they can’t reach the extremely high frequencies that Intel does.

Hopefully Intel will right this wrong, though in the current competitive climate we can only hope.

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