Contrary to popular myth, the G5 is not hot.

Everywhere I go on the the Mac sites, there is a declaration by somebody that the G5 is too hot to be used in anything other than Power Macs.

Hogwash. Let me lead you through some specifications:

G4 7455 (180 nm) 1.0 GHz: Typical 15 Watts, Max 22 Watts
G4 7455 (180 nm) 1.33 GHz: Typical 30 Watts, Max 40 Watts
G4 7447A (130 nm) at 1.42 GHz: Typical 21 Watts, Max 30 Watts

Of note, Apple publishes in press materials that the G4 7455 1.33 in the G4 Xserve puts out 45 Watts, so the Motorola/Freescale max number might be at bit low, but at least it's close.

G5 970 (130 nm) 1.8 GHz: Typical 51 W, Max ??? (est. 90-100)
G5 970FX (90 nm) 2.0 GHz: Typical 24.5 W, Max ??? (est. < 50)
G5 970FX (90 nm) 2.5 GHz: Typical 50 W, Max ??? (est. 90-100)

In contrast to Motorola/Freescale, IBM does not publish max power ratings. They only publish typical power ratings, and furthermore it should be noted that these numbers cannot be compared with Freescale's directly. However, these ratings are useful as a general guide. It has been estimated by hardware sites that the G5 970 at 1.8 GHz probably puts out more than 90 Watts, almost twice its typical rating. It is suspected that the 970FX at 2.5 GHz has a similar power output, especially given its identical typical power rating. If one were to double the rating for the 2.0 GHz 970FX one would get a max power of < 50 Watts, which is close to Apple's PR material number of 55 Watts. And measurements of the G5 Xserve have put the power output at less than 50 Watts running benchmarks at full blast.

OTOH, the G5 970FX (90 nm) at 1.4 GHz is rated for a typical power of only 12.3 Watts. We might thus guess the max power is < 25 Watts. Extrapolating, at 1.8 GHz it'd probably be close to about 30 Watts max, if the voltage could be kept constant (see below).

But wait, I've previously said that the 2.0 GHz 970FX is 45-50 Watts. What gives? Well, that's a desktop/server chip, and voltage is not a major issue. A deskop/server has effectively unlimited power. However, IBM has indicated that it views the 970FX an an all-in-one design from laptops to server. We already know that the 1.4 GHz chip runs at a lower voltage, and one might guess that there would be low voltage bins of the 1.8 GHz chips too specifically for laptops. Such low voltage chips would likely be more expensive, but that's OK in this context, since the power advantages outweigh the cost.

To get back to the original point, it seems from the specs that a G4 1.5 is hotter than a G5 1.4, and it's also probable that a G4 1.5 is hotter than a G5 1.5 as well. ie. MHz for MHz, the G5 might just be cooler than the G4, at least when we're talking embedded/laptop-oriented chip bins for the G5. Now who's to say IBM is going to make embedded/laptop chips? Well, Motorola for one, since they've been campaigning hard in developer documentation about their claimed advantages of the G4 over the G5. They wouldn't bother unless the G5 was not a real threat for their embedded bread and butter. Of course, I've also shown you the low voltage 1.4 GHz chip specs above, so clearly IBM is looking at this direction. And read on...

I have linked an interesting article in the title of this weblog. In the article IBM went on record to say that it feels that a 30 Watt chip is most appropriate for laptops, and suggests that this is doable up to 1.9 GHz. ie. I might be underestimating the coolness of the G5. And given that the G4 7447A 1.42 is 30 Watts, and that the PowerBooks run 1.5 GHz G4 7447A chips, it's likely that PowerBooks are already over 30 Watts. If IBM's statement is to be believed, this suggests 2.0 GHz 970FX chips for laptops are possible too, for a CPU power utilization relatively close to the 1.5 GHz G4 PowerBooks.

Now, Apple has already said that it won't release a G5 based PowerBook in 2004. If the chips are so cool then why not? Well, it's likely a number of reasons, not the least of which is chip supply, but one of the main issues here in terms of the CPU is not overall power per se, but power density (since the heat is more concentrated over all smaller die). (There is also the issue of higher power utilization of the G5's system controller, but that's a whole other kettle of fish.) But given the numbers above it's a reasonable guesstimation to think that the 2005 G5 PowerBook will reach 1.8 GHz. From the marketing standpoint, this makes sense too, since the PowerBooks are already at 1.5 GHz. A G4 1.5 --> G5 1.8 jump seems perfect.

BTW, this power density issue is also what necessitated the adoption of liquid cooling for the 2.5 GHz G5 970FX. Straight from the horse's mouth: Apple has stated the chip isn't any hotter than the previous chips, but that power density was higher.

So... There has been a lot of numbers here, but the take home message is that IBM has a very nice chip in its hands with the 90 nm G5 970FX, and at lower GHz speeds and low voltage, it's actually quite cool (in more ways than one). You can count on a G5 PowerBook in early 2005, running at speeds which even in 2004 are considered only the realm of Power Macs.