psu question

Is it ok to use a 800watt psu to power a pc that will use no more than 200watt.
I've put together a little low power computer out of mostly old parts i had laying around to use as a media server. Only PSU i have spare is a 800watt one i used in my main pc but i replaced a while ago cause i thought it had gone faulty (turned out it hadn't it was another issue that i later fixed). So yer is it safe to use such a powerful psu on such a low power usage pc?

It should be safe enough but the PSU may be very inefficient with such a low load. Do you really want to draw 800 Watts to run a small PC, especially if it's going to be on all the time?

It is safe, but unless the efficiency rating of the psu is really high, you're going to get a bit more power consumption.

In reply to a post by micksharpe:

It should be safe enough but the PSU may be very inefficient with such a low load. Do you really want to draw 800 Watts to run a small PC, especially if it's going to be on all the time?

It won't draw 800 watts all the time, it'll draw what it needs but do so with slightly more overhead.

In reply to a post by 12eason:

In reply to a post by micksharpe:

It should be safe enough but the PSU may be very inefficient with such a low load. Do you really want to draw 800 Watts to run a small PC, especially if it's going to be on all the time?

It won't draw 800 watts all the time, it'll draw what it needs but do so with slightly more overhead.

You're right. However, the efficiency of some PSUs can drop to 40% at low loads, even if they have a high efficiency rating at full load. For example, such a PSU could draw as much as 500W to run a 200W system. An "80 PLUS" certified PSU is guaranteed to be at least 80% efficient even at 20% load, and should draw no more than 250W to power a 200W system. The OP should check if his 800W PSU is 80 PLUS compliant, although if he is not paying for his electricity, he may not care.

...and yes, I got all this off t'internet.

Never occured to me about low loads being inefficient, you would think they would be able maintain that efficiency unless of course this is only in the really expensive ones.

Never occured to me about low loads being inefficient, you would think they would be able maintain that efficiency unless of course this is only in the really expensive ones.

The power dissipated by the PSU itself has load dependent and load independent components. The load independent component tends to be related to the total capacity of the PSU - say 2%(=16W) at 800W becomes 10% of the load at 160W - thus it's harder for big PSUs to be efficient at low loads. I wouldn't be surprised if the load dependent portion was relatively higher at low loads as well.

One thing to be aware of is that some switched mode PSUs need minimum current loads to maintain output voltages within spec. You should be ok with a well manufactured unit but be careful with older/cheaper designs.

I'd tend to avoid really cheap stuff though - too easily becomes a fire hazard!