Whether or not we're really in a 'post-PC' era is up for debate, but these are clearly changing times for the way we buy and use computers.

Mobile device usage may well be up, but even in these days of clouds, mobile phones and tablets, according to market analysts IDC the number of traditional PCs (desktop and laptops) sold every year is still growing, even in mature markets.

What is changing rapidly, however, is what those PCs look like. Thanks to ever less power hungry parts, thin and quiet Ultrabooks are on the up, while noisy, cheap and power hungry desktops are on the way down. It's a trend which favours small form factors and energy efficiency over the big old desktops of yore.

Which is why the new Xi3 from ISYS Technologies is an intriguing vision of the future. Shown off at CES earlier in the year, and recently at Design West, it's a small form factor desktop PC or server which - it's claimed - draws less than 20W when running flat out. By comparison, the average PC uses around 150W.

Even mini-ITX systems based on Via's C3 chip uses more energy, yet the Xi3 is built around an AMD Athlon 64 X2 processor. Not, perhaps, the most interesting of CPUs around or one you'd want to cut HD video on, but more than capable of running a Windows desktop and most associated applications.

And just as Ultrabooks and iPads take advantage of low power consumption to be creative and elegant with their design, so the Xi3 is a striking rethink of the way PCs are built. Measuring just over 10x9x9cm, it's capable of driving two 30inch monitors from a base unit that's around the size of a baked bean tin. Take that, Retina display.

In order to achieve this, ISYS has come up with its own, proprietary design for the internal components, which involves cutting the motherboard up into three distinct pieces. There's a board with the CPU on, one with the southbridge, and one for the rear ports too.

The Xi3 is a striking rethink of the way PCs are built
The Xi3 is a striking rethink of the way PCs are built

As a home PC, Xi3 is interesting, but all that clever architecture comes at too high a price. Starting at $849 it's impossible to justify over much more capable and versatile Acer S3 Ultrabook, for example, which only uses a bit more energy than the Xi3 but includes a screen. This isn't too much of a concern for ISYS as it has its eye on corporate sales and the data centre, where a few Watts saving per machine adds up to significant sums over five or six years.

To bolster its business case, a single Xi3 can be hooked up to three thin clients in order to provide virtual desktops for four employees.

These thin clients, called 'Zero Modules', cost less than $250 a pop and can take advantage of cheaper Windows licenses, but more importantly draw just 1W of power when on. So the argument put forward by ISYS is that you can have four desktops at a total power budget of just 24W. In a company with 2,000 employees, that's a potential saving of over 1,000kWh in a single seven hour working day.

Power efficiency on this kind of scale is a double win for corporates. Not only does it reduce running costs, it also helps them to meet environmental targets which are good for their image and help them meet guidelines for public sector contracts. However, the very inventiveness of the Xi3 may also be its undoing.

Xi3 price: too high - or better for other costs?

The biggest issue is still the price. There's a cost calculator on the website which shows the benefits over time for a corporate buyer, but there are some assumptions the company makes which seem unlikely. Firstly, that the Xi3 won't need replacing more often than a regular desktop, and secondly that all PCs are operational 24 hours a day.