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Do PSUs only use how many watts are required?

N

nLinked

Member
Making a small mini ITX PC with an Intel Atom motherboard. I can get away with a 65 watt PSU that comes with a case. But there is another case I like and it comes with a 200W PSU included.

Let's just pretend the total my computer takes is 40W. If I get the case that has the 200W PSU, will it still only use 40W?
 
From benchmarks I've seen, generally power supplies are at their peak efficiencies when the load is around 40-60% of the max wattage rating.
 
Yes, plus a bit more to cover the inefficiency of the supply itself. It's entirely possible that the 200 W PSU may actually be more efficient with the particular load you're putting on it.

Realize also that the total wattage rating is not the only thing to look at. The rating of the 12 V rail, for instance, or the overall quality of the supply, are very important.
 
One way to think of it is, if the PS did indeed put out 200W even when running a 40W computer, the extra 160W would have to go somewhere and would eventually fry the PS.
 
Think of it like a pipe with a valve. you will only pull as much as you open the valve till flow peaks and the valve is open all the way, minus the small leak we call efficiency.
 
nLinked said:
Making a small mini ITX PC with an Intel Atom motherboard. I can get away with a 65 watt PSU that comes with a case. But there is another case I like and it comes with a 200W PSU included.

Let's just pretend the total my computer takes is 40W. If I get the case that has the 200W PSU, will it still only use 40W?
Click to expand...

We should clarify: That 200W psu would still only GIVE YOU 40 watts. It will, however, TAKE more than 40 watts from the wall. For example, if your PSU was 80% efficiency at that load, it would TAKE 50 watts from the wall and then GIVE YOU 40 watts. However, if you are say way out of the sweet spot and the efficiency at that load was 50%, it would TAKE 80 watts from the wall then GIVE YOU the same 40 watts. This is why it is important to size your PSU correctly.
 
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