== Efficiency == 

 For reliable operation, it is important that the power supply be "big enough" for the application, not overloaded.  It is tempting to think "the bigger the better", but this is not entirely true.  Bigger will have a better margin of safety, should run cooler, and might run quieter.  Bigger costs more to buy, might be physically bigger, might not fit.  There may be a trade-off with efficiency.  Usually a power supply will be more efficient at the upper end of it's design range.  So an oversized PSU will be more lightly loaded, compared to its rating, and may be less efficient than a more appropriate small-but-adequate supply. 
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