Thanks Gagster,
It was probably too late when I wrote this post. (and likely this time also)
First: My real consideration is not with any particular system (or electronic device per say) but with the law of BIG NUMBER. Saving 50 Watts per computers might not seem a lot. However, many millions of them are delivered every quarter. Saving 50 Watts a million time's means 50 MWe or the equivalent of 250 Windmill rated at 1MWe. Even the alarm clock example where the saving is a mere 1W per units is likely to save 150 MWe (base load) or 1,314 GW/h per year in the Unite State alone. This is 750 Windmill rated 1MWe for the US alone (or many train of Coal).
Here MontainMan wanted to have a reason to invest in conservation: Easy, 150,000,000 Alarm Clock costing 1$ more because of efficient part = 150M$. 750 Windmill rated 1MWe costing 1M$ each to build: 750M$ + maintenance +rent of the land+insurance+Grid wheeling fee + lost in transformer+++.
Now back to computer. What I really meant to say is "why a regular Pentium 4 (usually use in desktop) consume 4 time more energy than a Pentium M (usually use in Laptop)." Now, the Pentium M is also use in "Blade Server" and some "Near Silent Desktop", and yes, evens some credible and usable gaming machine. (And you know that some laptop use regular P4 - But this is rare now)
I don't think that comparing AMD and Intel is appropriate for my line of thoughts. I wanted to indicate that if an "Electronic Manufacturer" did invest the needed money, they can easily reduce the "power consumption" of the electronic. P4 and PM are the result of two major (almost independent) investments from Intel. The PM needs to be energy efficient when the P4 does not. This is a "design" criteria that the engineering department has to fulfill or not.
Not every body purchase the "top of the line" Pentium 4. Many people purchase Celeron-D 34x or 35x that consume up to 73 Watts (Only for the processor). Furthermore, great many companies does purchase Pentium 4 3.0 GHz or less since most business application are perfectly happy with them. Saving 50 Watts of energy on the processor alone is not unrealistic in many scenarios.
I invite you to check this link: http://www.pugetsystems.com/max_pc.php, including the page #3 on power consumption of a credible "desktop" running on Pentium M. Take also note that the PSU of 600 Watts used in this benchmark is "ridiculously" big for a Pentium M that is unlikely to exceed 200 Watts under the most severs condition. When I don't have any benchmark data, the Enermax PSU 600 is unlikely to be particularly efficient with less than 1/3 of it rated capacity. If you want more information, google a bit and you should see more of that.
My own experience is with Blade Server, Gigabit Ethernet and Fiber Connect disk (not really Laptop, I know). Under these conditions, those Pentium M are incredibly effective on a "per watts" basis and we can stack many in a single cabinet (many more than any Xeon technology for sure)
Now that every one is gone because of the boring post, I will give you a scenario and a little bit more information about the Intel Processor. We can make comparisons using current processor technology, available in store and using the same 90 nm process. Consequently, comparing the Pentium M 780 where Intel has put the emphasis on "reducing electric consumption" and the Pentium 4 540, where the emphasis is clearly on "the highest performance possible" (well sort of).
Now, according to many benchmarks I have seen and some that I have performed myself, the performance difference between those 2 chips is usually minuscule, even in gaming situation. There are cases where the Pentium M 780 is clearly faster then the Pentium 4-540. But you may also disprove it. For example, the Pentium 4 is faster to handle MP3 and editing video content (where HT shine).
You were right that the Pentium 4 has fever transistor. Here is the data I gather: Willamette 42 M Trans., Northwood 55 M Trans. , Pentium 4 540 is 125 M Transistors and sized at 81 mm2. Finally, the Pentium 4 640 (the current Prescott 3.2 GHz 2M Cache) has some 169 M Transistor and 135 mm2 area.
Consequently, you are right when you indicate that a regular Pentium 4 have slightly less transistor. About 15 Million less so I make a mistake there. The Pentium M Dothan, is sized 84 mm2 - ~140 million transistors. Consequently, the PM is slightly larger than a Celeron-D 346 or the Pentium 4 540 but smaller than the Pentium 4 640.
Power consumption max:
Pentium M 780 : 10.8 Idle to 27 W max
Pentium 4 540 : 84 Watts
Pentium 4 640 : 84 Watts
Celeron-D 346 : 73 Watts
The same reasoning can be done with the Graphic Accelerator. For example, I own an Nvidia 6600GT and I am perfectly happy. So far, all the games I play do play good enough for me. May be, I should have purchase the Nvidia 6800??, I don't know. Nevertheless, I am convince that NVIDIA, if it invest it money for the goal of saving energy, can reduce the power consumption of the 6600GT, may be by half, and still provide the same gaming performance. I don't know. Furthermore, great many systems don't have a 3D accelerated card anyways.
In most house connected to the grid, there is several tens of devices that can save 1 Watts or more without modifying any of their function. Many houses have several TV, VCR and DVD Players. A sound system, a microwave, a door bell, one or many alarm clock, a computer or more, an ADSL or Cable modem, a printers, a few cordless phone, etc...
regard