To clear a few things up. A 600W PSU isn't going to be using 600W permanently. It will only draw how ever much power is required by the system. The efficiency, however, will change somewhat depending on the load. Here's an example of an efficiency curve over a typical loading range (around 20%-100% of maximum capacity):
And here's one that covers a wider range:
What we are seeing of both these units is typical - The efficiency varies a bit over the 20%-100% loading range, but not all that much. It drops rapidly as we go below about 20% loading, however (but keep in mind that the difference between 60% efficiency and 80% efficiency at say 100W AC draw is only 20W).
The message here? So long as your idle consumption is going to stay above about 20% of the PSUs maximum power output you can essentially ignore this. If you're planning to get a very high wattage PSU on an older system, or one that has very low power consumption, you can expect reduced efficiency. For the vast majority of people, though, this won't be a problem - very few people are going to buy a 1000W PSU to use on a P3.
Final Note: I'm oversimplifying this a little for the sake of readability - efficiency is also affect by a number of other variables including (among others):
- Relative loads on each rail (crossloading will reduce efficiency)
- Temperature
- Input voltage (230V input will always produce higher efficiency than 110V)
As for sleep power consumption, it's quite low - in the order of 50-80W for a typical PC. If you're interested you can buy a wattage metre such as a
Kill-a-watt and check your own PC.
