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Does a higher-wattage PSU automatically draw more power in comparison to a lower-wattage PSU even if the components fed by the PSU remain the same?

The reason I'm asking is that I'm thinking of getting a PSU with more wattage than currently necessary for my server, to make expansion (e.g. additional CPU) later easier without having to exchange the PSU.

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up vote 3 down vote accepted

Does a higher-wattage PSU automatically draw more power in comparison to a lower-wattage PSU even if the components fed by the PSU remain the same?

A bit, yes.

Computer power supplies typically become more efficient the closer they are to their rated capacity, so if you're running a large power supply with a low load, it's going to be less efficient than a properly-matched power supply.

How exactly this translates into power/cost savings, though, is highly dependent on your environment.

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The difference is 570W (enough for now) and 870W. Is that considered a large power supply and you think the difference is most likely dramatic? I'm hoping not to get switched to the next tier with our data center just because we're changing the PSU. – AX1 Dec 10 '10 at 21:58
    
@AX1 The difference can be between 81% and 89% efficient for low and high load. If the 870W is working at 81% and the 570W is working at 89%, at 90% load for the 570w power supply the difference is is about 56W. The actual spread may be smaller. – sysadmin1138 Dec 11 '10 at 1:29

Absolutely not. 30% reserve is a must for a power supply. Technically, larger the load, less stable the voltage. Efficiency depends on the unit model, circuit. The perfect power efficiency is neither at max nor at min load. That is not a thing you should care about. The life time and voltage stability is much more important. You can wonder, why o we install 700W PSU in gaming i7 systems and only 350W for server. It is simple - periphery and workload. While servers provide great I/O and redundancy, the power consumption is not so large because less math is executed in general scenario. Xeon CPUs are much more energy conservative than their high performance little i7 brothers.

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The more hard drives you have the more power you use the rest of the components don't use that much power.

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This isn't even what the guy is asking... – boatcoder Dec 11 '10 at 1:27
    
thanks Mark0978 but it was answered before - just pointing out that hard drives use most of the power - so is basically up to the user environment (as answered ...) – silviud Dec 11 '10 at 2:09
    
Say what.. That may be true if you have a lot of drives but at ca 5-15W per drive vs 50-120W for a loaded CPU it takes a lot of drives in a busy server to take the title of power hog. – Helvick Dec 11 '10 at 2:20
    
Helvick you are right - if you just have one or two hard drives they will that much power but if you have lots of them they will be very expensive, also you have to consider memory approx 10 per bank. for the original q - my answer is NO - having a bigger power supply doesn't mean at the plug you will use more. The power rating on your PSU is the sum of what the different transformers inside can supply, so don't trust it blindly. Further to say if you use your computer for high computations you will consume more regardless of the power supply. to get real stats use Kill A Watt. – silviud Dec 11 '10 at 3:08
    
Silviud, your part about a bigger power supply not using more power is not necessarily true, as Erik explained and the rest is simply obvious. – Sven Dec 11 '10 at 4:17

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