Consider a laptop.

Laptops use the least amount of energy and contain the smallest amount of material.  Their built-in batteries eliminate the need for a separate backup power supply, which can also consume additional energy.  Their built-in screens also tend to be smaller than desktop computer monitors, so they save energy there as well. TopTen recommends buying a laptop if your performance needs are low or moderate and you want to minimize total energy consumption.

If portability is not important, look at all-in-one or mini (“small form factor”) options.

All-in-one computers have similar performance to a laptop at lower cost, or a little higher performance than a laptop at a similar cost.  Like laptops, they have limited expandability options and built-in screens, so only need one power supply to run the computer and the screen.  We would expect their energy use to be a little higher than that of a laptop, but still very good for people who don’t need the battery-operated portability of a laptop. Small form factor computers do not include built-in screens, and generally offer little if any expandability, but they can be very affordable and have very low energy consumption, while being nearly as portable as a laptop.

Skip unnecessary bells and whistles.

Full-sized, “tower” computers are not all the same. All of the add-ons they feature or are built to accommodate--optical drives, huge hard drives, or more powerful video cards—are energy drains. A powerful video card can double the energy consumption of a desktop PC. We’ve found that many desktop computers have power supplies much larger than they actually need, just in case the user upgrades everything inside the box to the maximum possible performance or capability.  Be realistic about your likely expansion needs and you can save some of the energy use and cost that comes with an oversized power supply.  Unless you need multiple video cards and processors and a very elaborate cooling system, you should be fine with a power supply in the range of 200 to 300 watts.  For more information about locating a highly energy efficient, properly sized computer power supply, see www.80plus.org.

Don’t expect performance to exactly match the testing data.

It is not possible for manufacturers to test every single combination of options (memory, processor type, hard drive, optical drives, video cards, etc.) when reporting their data to ENERGY STAR, so the exact configuration you purchase may have slightly different energy use than the estimates reported here.