There are some non-obvious changes to the standard as well. The output load capacity was adapted to the latest hardware requirements. There is a second +12V voltage slot for the CPU, so the other 12V current doesn't get overtaxed if there is a sudden surge in the CPU load.
Efficiency was notched up considerably, with the new standard 70% at peak and typical load, and 60% at lower loads. The recommended efficiency rates are as high as 75%, 80% and 68% respectively for peak, typical and lower load. (A typical load is defined as 50% and low capacity as 20% of overall output.) While an efficiency value of 70% looks pretty high at first glance, consider that it means the PSU itself consumes up 30% of the power, which it blows out of the case as waste heat. If the processor is not running at a high load, the percentage of unused power is even greater; at 50% this means fully half of the input energy dissipates as waste heat through the PSU. In light of this, it makes sense for users to think about whether they really need that huge 550 or 600 watt power supply - maybe you can get by with less power. It's also worth noting that the enormous amount of power used by today's CPUs makes the efficiency of the PSU dwindle somewhat into insignificance.
With a typical PC configuration using a Q-Technologies 460 W power unit, we measured the power consumption of the entire system at 195.8 watts, while the supply took a total of 247 watts. This works out to an efficiency of 79.3%, so the power unit is putting out some decent numbers here. Still, even here, over 50 watts still leave the unit as heat rather than contributing to computing. If the power supply has an even lower efficiency rating - no rare occurrence, by the way - you're sure to see the effect of the much higher power output on your energy bill. Simply put, an efficient PSU may cost more initially but can save you cold, hard cash, especially if the computer is left running all the time.