Energy cost has gained more and more importance over the last few years, motivated both by the rising cost of electricity and ever increasing power consumption of PC components. For our calculations, which project running costs of the individual systems for a year, we assume a price of 18 Euro Cent/kWh.

AMD remains the champion when it comes to energy costs when idle. Practically all of Intel's processors have to line up behind their rivals from AMD, with the new L2 stepping as the sole exception.

Under full load, the ranking changes quite a bit, though, and Intel is able to outperform several of AMD's entries. Only the Sempron CPUs use less power than Intel's Core 2 Duo E6300 with L2 stepping. Nonetheless, the processor is not the sole deciding factor with regard to the entire system's power consumption. Additionally, the chipset and the voltage regulators for the processor also influence the overall power consumption. As mentioned before, we'll cover this in more detail further on in this article.

Energy Index: AMD Unbeatable

People tend to forget one thing when calculating how much an energy efficient system can save them - namely the processor's price. After all, saving a few quid by using an energy efficient processor that cost ten times what you saved is quite the hollow victory. That's why we've calculated an index that takes into account both the processor price and the energy cost.

The processor with the lowest retail price is awarded 50 points, while the most expensive one gets 0. We apply the same method to the energy cost, rewarding the lowest power consumption with 50 points. Conversely, the most power-hungry processor gets naught. The sum of these two sub scores yields the overall index, with a maximum score of 100.

At this point, a processor's performance does not factor into the equation. We will go on to evaluate it later when we look at the efficiency measurements.