Tanarus wrote:
Your computer will only draw what wattage it needs from the power supply. If your tower only draws 300W from a 600W power supply, then your computer will only be consuming that 300W.
However, some power supplies are more efficient than others at certain operating levels. Choosing the correct power supply is important from an efficency point of view.
Here's a bit of info from an article from SilentPCReview [http://www.silentpcreview.com/article28-page4.html
CORRECT PSU SIZING FOR BEST ENERGY EFFICIENCY
This was a topic of discussion in one of the sessions at the Spring 2005 IDF in San Francisco: the concept of choosing a PSU whose efficiency curve is well-matched to the system power load. Such matching can yield incremental improvements in average power consumption and ensure minimum waste heat generation. Correct PSU sizing is very carefully practiced by tier one computer makers concerned with maximum cost effectiveness.
SPCR's own PSU testing has shown that power efficiency in PSUs varies with load, and the load at which the best efficiency is reached varies from model to model. In a given line of PSU models, the maximum and average efficiency tends to be very similar; where the peak occurs depends on power rating. Most PSUs reach peak efficiency between 50~75% loading, tail off a bit at maximum power and drop at least 10% at minimum load.
PSU efficiency data from SPCR review database.
The vertical scale has been truncated for clarity; please see text below for full discussion.
A system that draws ~250W maximum and idles at <100W would be a good match for the 300W PSU shown above.The efficiency power curve of the 600W PSU is better suited for a system than idles at >150W and peaks at >300W. It would be a substantially worse match for the system of the previous example, as the PSU would be operating at a mediocre <75% efficiency in idle, and only just reaching 80% at peak.
From a PSU heat waste point of view, the differences are significant:
At 200W load,
- the 300W model would generate 44W of heat (18% of 244W AC input);
- the 600W model would generate 50W of heat (20% of 250W AC input).
At 90W load,
- the 300W model would generate 23W of heat (20.5% of 113W AC input).
- the 600W model would generate 32W of heat (26% of 122W AC input).
Using the 600W PSU with this system is an example of incorrect, costly PSU sizing. It is practised most frequently by gaming enthsiasts who are encouraged to believe that greater power capacity is always better. Whether 480W, 550W or >600W PSUs are suitable for system that cannot possibly draw even 250W is a type of question asked almost daily in the SPCR Forums.
