Desktop vs Laptop

Notebooks are the most obvious way to save energy in home computing and client-side office computers. Energy consumption is a critical factor in notebook design. It determines how long the batteries will last, which should at least be a couple of hours even with the most power-hungry processor. As a consequence laptops use the most energy efficient displays (LCD), adapters, hard disks and CPUs that are available. All CPU manufacturers have special mobile versions of their CPUs sporting PowerNow! (AMD), SpeedStep (Intel) and LongRun (Transmeta) power-management features.
Comparing the 30W for a powerful notebook (including LCD screen, see example) with the 120W desktop PC plus the 80 W CRT screen, savings could really be up to 80%. And even in the notebooks intended as 'desktop replacement', with a larger screen (up to 16-17") and less aggressive power management settings, the savings are still well over 50%.
In areas with blackouts and power-surges the notebook (batteries included) could save even more. In these area's desktop PCs would typically require the backup of a UPS (Uninterruptible Power Supply) to keep you from losing data. And a UPS is not only a significant extra cost, it is also a significant energy-eater.
All in all, even when only looking at the energy cost advantages, buying a notebook might well be worth the extra money. With the ENERGY STAR database, it is simply a matter of doing the maths, taking into account intensity of use and expected product-life (typically 5 years).
Having said that, there will be plenty of areas where the expandability of the desktop PC is a must. Game enthusiasts and CAD engineers will often want to replace the graphics card. When using the PC for small server or home server applications, the desktop allows easy expansion of hard disk capacity or a few extra ports. And of course, if your budget takes priority over total cost-of-ownership, most desktop PCs are cheaper than notebooks.

Example

Notebook Energy Use
A 2002-model laptop with a Mobile Athlon XP 1500+ processor uses around 20.5 W in 'desktop' mode. Of this some 3W are for the CPU and some 7W for the very bright (160 cd/m2) LCD screen. Chipset, graphics, memory and power supply(adapter) take up a little more than 10 W. At 'full load', the laptop uses 47 W, mainly due to CPU use: 36 W. Average energy use is estimated at 20W, which means 2 hours with a 44-Wh Li-Ion battery. [source c't 2002, no.10]
In PowerNow!-mode, with 500 MHz and 1,05 V, the energy use of 36W could be tuned down to 20W at full load. Intel's ULV(Ultra Low Voltage) Pentium III-M tops that with 7.5 W at 800 MHz and a mere 3.4 W in SpeedStep-mode. Similar values are reached by low-power champion Transmeta with its Crusoe chip. Typical standby modes of these laptops are lower than a few Watts.
Through all this attention for the CPU, it may seem that this is the key factor in the energy consumption. However, Intel measurements of a Pentium 4 notebook using the ZD BatteryMark benchmarks, which simulates normal office use including all the micro-pauses, shows that it is the platform (chipset, graphics, power supply) that counts the most and that is where the potential lies.

Basis: SpeedStep enabled Mobile Pentium III, dissipating around 12 W on average using the BatteryMark 4.01 benchmark test, which simulates real-life office behaviour of the user (presentation at Intel Developers Forum 2002). Note: Recent data with the Pentium 4 shows that CPU goes from 7 to 10% and the chipset from 13 to 15%. [source www.extremete