Apple's Mac Mini might be one of the more expensive mini-PCs on the market but consumers will claw back some of that in cheaper running costs, new figures from the energy website have suggested.

Not surprisingly, small form factor and ‘net-top' PCs dominate the energy efficiency list, with only one conventional desktop making it into the top 26 places. Apple claimed the top four places for its Mac Minis, with the best system being the entry-level Core 2 Duo-based model, quoted as using 12.6 watts while turned on but not under load, or 1.71 watts in sleep mode.

Further down the list came the Asus Eee Box B204 running a 1.6GHz Intel Atom N270, which consumed 13.71 watts in idle and slightly bettered the Apples in sleep mode, using only 1.35 watts. Most of the rest of the PCs in the top regions of the list use between 15 watts and 30 watts in idle mode.

How much would this frugality add up to in a day, or a year? Assuming the UK national electricity tariff average of 13.94 pence per unit, they quote the Mac Mini as costing 3 pence per day in 'idle' mode, or £6.89 per year in the most efficient model, compared to the £7.52 per annum of the Asus B204. The annual carbon emissions for the Mac minis were around the 21kg mark.

"There's a staggering difference between the energy consumption of computers in the marketplace. You could save nearly £150 per year on electricity in choosing the most efficient models," said sust-it's Ross Lammas.

How ‘staggering' will depend on which system these are compared to. A fair example is probably the Dell 545 PC, which runs a 2.66GHz Core 2 Quad, eating 70.93 watts or £35.45 per year. That adds up to £29.91 more than the Asus, though it is worth pointing out that anyone used to a Core2 Quad PC would probably find the Atom N270 system severely underpowered at any cost.

That points up a remarkable aspect of the performance of the Apple systems that deserves to be underscored - they all use mainstream Intel Core 2 Duo processors rather than more recent low-power designs such as the Atom. The implication of this is that the power-saving of the current Atoms has been exaggerated compared to desktop processors probably because other components such as memory, video and hard disk also consumer power and these are unavoidable in any PC.

It is also possible that the 'idle' power quoted is misleading and the Atom would scale to real-world use without drawing as much electricity as the Core 2 Duo. The statistic is still surprising, something acknowledged by Lammas himself, who pointed out that all consumption numbers were quoted from the US government Energy Star website.

In terms of price, Apple's Mini systems typically cost from two to three times that of other small PCs on the list from PC rivals such as Acer, Asus, and Fujitsu. As an example, the Asus Eee Box B202 can be found for around £230 including VAT in the UK, while the admittedly more powerful Mini costs from £499.

The final calculation on which computer is the cheapest to use will also depend on the monitor and peripherals chosen to work with the PC, and separate figures from sust-It show a wide variation from around 13 watts in use consumption up to 40 watts or more assuming a 19 inch LCD screen is selected.