HARTPURY, United Kingdom — Apple Computer has the most- and least-efficient computers on the market, according to rankings released by the website sust-it.net.

Across three categories -- laptops, desktops and all-in-ones -- Apple scores at or near the top of the list, and is also at the bottom of the list for its desktops and all-in-one iMac.

For laptop computers, Apple's MacBook Air scores the highest, using just 1.07 Watts of power during sleep mode and just .56W when turned off; following the MacBook Air is Hewlett-Packard's HP Pavilion DM3, which uses .95W while sleeping and .59W while off.

In desktops, Apple's Mac Mini took top honors, using between 1.71 and 1.82W while sleeping and .94 and .96W while off, depending on the size of the processor. Asus's Eee Box B204 took second place, using just 1.35W while asleep and 1.11W while idling.

The much-smaller all-in-one computer category was where Apple failed to shine: its 2.66GHz iMac was its most-efficient machine, landing in sixth place. Tying for first place were eMachines' EZ1601 computer and the Acer Veriton Z280G -- both used .99W while asleep and .63W when turned off.

In addition to ranking computers based on their energy efficiency, the site also calculated how much each machine cost to own for a year, based on the Energy Star Typical Energy Consumption usage figures. The best-performing laptops used about £3.00 (about US$4.99) per year in energy cost, while the best-performing desktops cost an additional £6.89 (about US$11.46) per year in energy usage.

Inefficient computers, however, had a much higher cost to their owners: while the least efficient laptops (HP's Pavilion DV6 and DV7T and Panasonic's CF-52E) used between £8.30 (about US$13.81) and £7.73 (about US$12.86) per year, or nearly 300 percent more, the worst-performing desktops were major money-suckers as well as energy suckers: The Fujitsu Siemens CELSIUS R670 desktop cost an additional £155.73 (about US$259.08) per year in energy costs -- over 2000 percent more than the most efficient machines.

Another model in Fujitsu Siemens' CELSIUS line came in second-worst with £130.88 (about US$217.74) in annual energy use, while the Apple Mac Pro desktop is the third least-efficient desktop, costing £107.45 (about US$178.76) per year to power it.

The gap between best and worst all-in-ones was narrower, with Apple's iMac using £33.11 (about US$55.08) per year in energy and HP's TouchSmart 9100 using £25.21 (about US$41.94), a 450 percent increase in energy costs.

Sust-it.net compiles listings for a host of different electronics, from computers to washing machines, and offers energy costs for comparison across the U.K., Europe and the United States.