Do psus draw their wattage or just what they need?

[minicale]

This is most likely a stupid question but say I have a 650w Psu would it use 650w all the time or would it only draw what it needs to power the computer because my friend won a 650w psu that he will sell me for a good deal but I only need/want a 500-550w so I would be getting 100-150w then I want and I don't want it to add to the electric bill when I am not utilising it all.

[AETAaAS]

A PSU will only use what components need on top of whatever inefficiency.

If your computer is idling for example, and the components (CPU, GPU, drives, etc) use only 100W but you have a 500W power supply, you may be drawing 120W from the wall (the extra 20W coming from inefficiency, hence the 80+ ratings on PSUs) you will not draw the 500W the power supply is rated for.

[kompukare]

Simple Answer: Only what they need.

More complex answer: PSUs have differing efficiencies depending on their load. So for example a 1000W PSU might be 80% efficient at 1000W (i.e. it would draw 1200W), but at say 10% load (that is when drawing 100W), it might only be be 60% efficient (i.e. it would draw 166W).

Generally the difference is not that big (for instance this random jonnyguru Seasonic review has the efficiency staying at 89% even when the load is only 135W). But that's where there is a potential for wasting power if you use a PSU which is too powerful.

You should be fine.

[minicale]

So I wouldn't notice if I swapped my 500w psu to a 650w? Would it cost the same to run?

[kompukare]

So I wouldn't notice if I swapped my 500w psu to a 650w? Would it cost the same to run?

The only way to accurately answer that would be if were to tell us the exact models of each PSU so we could look them up in a proper review (assuming those PSUs have had a proper review of course). But yes, the difference is likely to be either very little, nothing or even cheaper to run (if the 650W is more efficient than your old 500W one).

[AETAaAS]

Depends on the efficiency of either one and your use case. As kompukare pointed out, excessively powerful PSUs can be worse in normal use cases since the 80+ rating is tested at 20,50 and 100% of the PSU load. (There has recently been the introduction of the Titanium rating which tests at 10% but is exceedingly rare if it exists and will certainly be eye wateringly expensive)

Idling computers frequently use less than 100W and hence PSUs are not properly rated for this, if they are 650W for example because the lowest tested efficiency (20%) will be about 130W. In active use however, computers enter the rated efficiency loads.

The easiest way to check it out is if your PSU is rated higher on the 80 Plus ratings.

[kalniel]

So I wouldn't notice if I swapped my 500w psu to a 650w? Would it cost the same to run?

It depends on the efficiencies, as pointed out above. If for the same load, they had the same efficiency, then yes, it'd cost the same to run. 650W isn't a crazy high supply so you're probably fine. Where it causes problems is when someone thinks they need a 1000W supply but they're using modern components which hardly draw anything - the power supply sits in such a low range that it's actually less efficient than a smaller supply would be. The very latest (and best quality) PSUs are getting better and keeping efficiencies high even for low loads though.

[minicale]

Yeah the one I am using is a Silverstone Strider 500w 80plus and the one I am going to get is a 650w modular Seasonic 80plus bronze (main reason I want it because its modular).

Thanks for clearing that up for me and responding so fast this is one reason I like the hexus forums so much.

[mikeo01]

Depends on the efficiency. A gold certified 650W PSU may run more efficient than a standard 500W. As mentioned above, you may be wasting power because the PSU has to convert for your components.

So it's best practice to purchase a power supply based on your watt and amp requirements and add 20-30% more to that for inefficiencies. Look for at least bronze certified PSUs if you're concerned.

EDIT: As mentioned above

[kalniel]

So it's best practice to purchase a power supply based on your watt and amp requirements and add 20-30% more to that for inefficiencies.

NO.

The watt given on PSU ratings are the *supplied* watts - ie efficiency doesn't come into it.

A 90% efficiency 600W supply will supply exactly the same watts to your system as a 50% efficient 600W supply. Literally no difference. But that latter will draw more from the wall.

But you are right that you want to spec for ~20% over peak expected load, but that's only because PSUs tend to drop in efficiency a bit when at 100%.

[mikeo01]

I'm saying if your requirement is 500W and you purchase a 500W PSU, you're going to overload it because not all power supplies can actually output their rated wattage.

[kalniel]

I'm saying if your requirement is 500W and you purchase a 500W PSU, you're going to overload it because not all power supplies can actually output their rated wattage.

Well you should never buy such a supply in the first case, if there's a danger that it can't make it's rated wattage then it's junk.

[mikeo01]

That's what I mean.

A 50% as you said will draw more from the wall making it less efficient whilst a 80% supply will require less. I worded it stupidly.

[Saracen]

This is most likely a stupid question but say I have a 650w Psu would it use 650w all the time or would it only draw what it needs to power the computer because my friend won a 650w psu that he will sell me for a good deal but I only need/want a 500-550w so I would be getting 100-150w then I want and I don't want it to add to the electric bill when I am not utilising it all.

It's sort-of a bit of both.

It'll draw what it needs, not 650w all the time, BUT .... psu's aren't 100% efficient.

There is a loss between the proportion of power going in, and the proportion of power coming out, and that ratio tends not be be linear across the whole range.

In a somewhat simplistic explanation, let me try to clarify that.

If you have a PSU that was 75% efficient in a linear fashion, then however much of a load you put on, the power out is 75% of the power in. The rest is lost in heat, etc.

So .... if you put in 100w, you get 75w out. If you put 200w in, you get 150w out, if you put 600w in, you get 450w out.

But you can look at each of those the other way round, which is, in the last example, if your system needs 450w out, to power CPU, graphics card, hard drives etc, you need to put 600w in to get that.

So, the efficiency of the PSU tells you the ratio of power out to power in. The more efficient the PSU, you less the power drawn from your mains to supply a given power to you system. So, if you need 250w for your system, a 60% efficient supply will eat 417w (that is, 250 ÷ 0.6) from your mains to do it.

If, on the other hand, the PSU is 90% efficient, the to get 250w out, it only eats 278w (250 ÷ 0.9).

So, both PSUs supply 250w to your system, but you pay for 278w from your power supplier with the 90% efficient one, and 417w with the 60% one.


But remember I talked about "linear"?

Power supplies are not equally efficient from supplying 1w to maximum watts, say, 650w. A PSU might be 90% efficient when drawing 80% of it's load, but might only be 35% efficent when drawing 25% of it's load. So, you don't want a PSU running flat out all the time, but you also don't want a 1200w behemoth if you only need 150w.

In an ideal world, therefore, you want a PSU that is at it's most efficient when delivering the load you want most of the time, but still has enough headroom to deliver when demand peaks (such as gaming on a power hungry graphics card), and to allow for some future expansion.

My rule of thumb is that a PSU that is rated such that my system draws about 70% of rated power under normal load. That gives me headroom for heavy loads and expansion for the future, while still being at or near the point of optimum efficiency, which is usually around 75-80% of it's maximum.

So if my system needs typically 350-400w, then call it 375w. I want a PSU where about 75-80% of it's maximum is 375w, and that is between about 469w and 500w, so I'd go for something around 500w for the PSU, and get the most efficient I could afford.

The trick, therefore, is getting an idea of what your system needs now, and over the next few years, and getting an efficient PSU large enough, but only just large enough, to cope with that demand, plus headroom for gaming peaks, etc. Oh, and getting one that's well designed and built, too.

[Pob255]

I think Saracen has put it pretty clearly there, however another thing to consider is that the power draw is not linear ether, the power draw is a vary large range too, dependent on the components and usage.
Most relative basic systems can drop below 100w when idle often to around the 50w range, but a lot will depend on the components you use.
The 80+ testing means most psu's have efficiency curves where the top of the curve is in the 20-100% range, efficiency drops off below 20%, the issue is how quickly and how much?
Well there's no official numbers here, luckily jonnyguru normally does 10% testing too eg http://www.jonnyguru.com/modules.php...tory3&reid=338 here we see that a psu that is in the 83-86% efficiency range in the 20-100% 80plus testing range is only 75% at 10%

back to the op and original question, the only one who can really answer the question is you, you'd have to o some testing and see what the power draw is, just a simple wall plug in monitor (cost around £10-20) will give a good idea as to how much current your pc is pulling at both idle, load and general usage.

[davidt99]

Unfortunately most people purchase a PSU on price. Cheap psu's are generally poor performers.