PCs- Do larger power supplies use more power?

My brother and I were pricing PSUs for a PC we plan on building and we were going to buy a 380W PSU (enough for the modest PC we’ll end up with) and then I saw a PSU on sale of comparable quality for the same price but has 550W of power.

Will the latter PSU cause a higher power bill to be had than the former or will they both draw roughly the same amount of power because the PC parts being used are the same?

No. PSU’s only deliver the amount of power needed for the system to run. So if your components require 300W and you have a 500W PSU, then it will only take 300W. What matters is the power efficiency of the PSU and that differs from product to product.

But the question is “Will the latter PSU cause a higher power bill to be had than the former?”

According to the Wikipedia article on “Power supply unit (computer)”, the answer could be yes (depending on how much the total load of the PC is).


A bigger rated PSU will indeed consume just a bit more standby power. Like a motor most PSUs have the best efficiency at near their top rating. On the other hand this assumes both have the same efficiency which may not be the case.

Okay, the system requirements for the video card I want to get says “300 Watt or greater power supply”.

Since the Wikipedia article says “Efficiency generally peaks at about 50-75% load”, would I save electricity by going with a 500W or 550W rather than a 380W PSU (assuming all PSUs are rated as having the same efficiency)?

It’s hard to know without knowing how much is really going to be drawn. If you’re using a high-end video card and have a number of peripherals and drives, it’s probably best to ignore efficiency in favour of being sure your power supply isn’t going to be overdrawn and go up in smoke.

Look up the supply on the 80 plus website (you are buying one listed there, right?) and you can see the efficiency curves over a range of outputs.

To look at the worst case conditions. That is each PSU sypplying max wattage it can provide. And lets assume $0.15/KWH over a month.

380watts 24/7 for 30 days is 273.6 KWH cost 41.04 550 Watts 24/7 is 396 KWH Cost 59.40 increase of $18.36

But if the load on the PSU is under 380 watts then the 550watt unit will probably be just a little lower efficent.They may have the same efficency through out the load. But if you are running the load on the PSU for long pierods of time around 380 watts the 500 watt PSU will have a longer life.

And when you peak over 380 watts with 550 watts you should not have a voltagfe drop.

It is extremely difficult to compare two power supplies on paper. The only way would be to make actual measurements at diffrent loads. On the other hand the difference in consumption will be quite small and will not represent much in terms of cost.

What about the claim from the Wikipedia article that “a power supply that’s over twice the required size will be significantly less efficient, and waste a lot of electricity”?

The article also claims “Efficiency generally peaks at about 50-75% load”.

Since the video card I want says “300 Watt or greater power supply”, wouldn’t I save the most money going with about a 500W PSU as it’s not double the requirement and not just meeting it either?

Here is the video card I plan on getting:

Yes and without knowing what specific models the OP is thinking of purchasing that is impossible to determine. Whilst I believe the wikipedia article is correct, in practice purchasing a 500W PSU may not be necessarily less efficient than a 380W.

Your best bet is looking for user reviews on the PSU, finding the actual efficiency results. The 500W PSU could feasibly be more energy efficient. What we would require is the exact model to look into how efficient it is.

You would have difficulty getting the efficiency graphs for both power supplies at different loads and even then I would not trust them much. I would only trust actual measurements but that means you’d have to obtain both power supplies and test them. So, just for discussion we will have to assume that your best bet for efficiency is to use a power supply which is running at something like 80% of full rated load capacity. Except that your load is also very variable and difficult to determine without doing actual measurements. And that if you add a board or other component your load will increase.

Me, everything else being equal, I would just go with the larger PSU.

But x-ray vision’s point was that you answered “No”. :dubious:

As far as what models I’m considering, I already posted that the question assumes that the models have the same efficiency rating. What I want to determine is assuming that various PSUs have identical efficiency ratings, what wattage PSU will give me the best power savings and is the claim from the Wikipedia article that “a power supply that’s over twice the required size will be significantly less efficient, and waste a lot of electricity” an accurate claim.

Okay, I didn’t realize it would be so difficult to determine the load.

I’m guessing you’re thinking the 380W PSU isn’t as good an idea as a 500W?

Everything else being equal I would go for the larger one. Execept that everything else is probably not equal. Computer PSUs are mostly cheap-built and unreliable. I would go with a smaller one of good quality rather than with a bigger one of crappy quality. PSUs can only be sold so cheaply mostly due to false advertising. Those “watts” they advertise are more like “miniwatts”. I have seen some really crappy PSUs out there. On the other hand, mostly they just blow out and you can get a new one. rarely would they damage the computer, although it can happen. I have repaired dozens and they have crappy components, mostly crappy capacitors. Bad caps is the main cause of PSU failure.

By the way, I have done quite a few measurements and have never seen any computer draw those hundreds of watts. I regularly leave a computer on for months at a time so I can log into it from around the world. It has never occurred to me to check if I could save a few pennies by changing the power supply. My computer (with monitor off) consumes between 110 ~ 135 watts and it has several cards.