Power Supply Max Load Capacity

Question

How do you find out what a power supply max load capacity is?
What is a max load capacity?
Should this be listed somewhere on the computers specs?
Do all power supplies have a max capacity over the listed wattage (eg. 400W Power supply has a max capacity of 505W)?
Is there always a max capacity?

Anything you can tell me about the max capacity of power supplies would greatly be appreciated.

Bobman101 · Answer

Not all have a maximum over their specified rating, becuase as far as I know, most actually are rated by their maximum (or in the general area of it). The Dell PSUs are the one exception that I know of.

I'm unsure of the maximum for the 250 watt PSU in older Dells, but I had my existing setup on one for about a month or so. I don't know if it would had held up long term like that, which is why I upgraded. The other time I loaded a Dell was when I had the folowing a the Dimension 4100's 210 watt PSU: PIII 800MHz CPU, 2 sticks of RAM, 3 HDDs, 1 CD-Rom device, GeForce4 Ti4200 video card, sound card, NIC card, and an internal modem card.

x_lab rat · Answer

I tested power supplies, but not for 'max load'.  How much excess capacity is built in over and above the ratings is not a standard amount like '25%', it's up to the designer.  The tighter a price point he's trying to hit, the closer the stated rating is to absolute max.  Running anything at its absolute max rating can shorten its life.   A label on the computer rates the power consumption as Amps @ Volts.  Multiply these two numbers.  That is the consumption including the loss within the supply itself, running at rated load.    The label on the power supply rates output the same way, A @ V, but for each separate rail (at least 3--12V, 5V, 5V standby).  A x V = W then add the Watts.   Peripherals (drives, cards, fans) sometimes list their loads.  Example, a typical CPU fan draws 1 Amp @ 12V, for 12 Watts.  A harddrive lists the startup load, which only lasts 1 second, and it's much (2~3x) more than the run load.  Peripherals requiring greater-than-nominal power (performance video cards) will state the recommended power supply rating, using an average value for loads other than itself.   A known-quality brand of power supply has 15~20% more short-term capacity than its rating, but that load level is not meant to be sustained much beyond startup, ~1 second.  A cut-rate power supply may not have any excess capacity to speak of.

c_arsena · Answer

'The Dell PSUs are the one exception that I know of.' How do you know this and where do you get this information please. :) x-lab: So Amp x Voltage will give me wattage? If I open up my case and look at my power supply will is have the maximum output? Or am I just supposed to guess what the maximum output is. I ask because, isn't it misleading if the maximum output is not listed anywhere for the consumer?

x_lab rat · Answer

Yep, Volts x Amps = Watts.  Google it.   99% of consumers don't know what Watts are and don't care.  No matter what laws say what a vendor must disclose on the label, vendors will find a way to cheat it.  Does anybody get the government mileage rating on their cars?

c_arsena · Answer

So if I write down all the numbers on the side of my actual power supply and do the math, the numbers that I come up with cannot be exceeded by the power supply no matter what the maker says?

x_lab rat · Answer

The numbers on the PS label are what the maker says.  There is a test to determine what the actual limits are but it requires highly specialized equipment.

c_arsena · Answer

but the numbers on my PSU say MAXIMUM 305W and dell tech support is telling me that the max is 415w (give or take 10w)

x_lab rat · Answer

Sorry, can't accurately explain that without testing the supply myself.  Rating methods vary, we kinda covered that.   What's the issue?  Can you use a video card that says '400W power supply'?  Likely get away with it.    Upgrading as a practical concept has been made awkward, on purpose.  Vendors would rather sell new systems than piece parts.

c_arsena · Answer

I using a video card that requires a min of 350W. My games freeze, I get runtime errors, vital call errors and game exceptions. This particular game demands a lot from the video card, more than most applications, and I think that the video card is demanding more power from the power supply than the power supply can give it, thus, my problem.

x_lab rat · Answer

It's possible power is sagging (dropping below rated voltage)--this is known as foldback, where the supply reduces voltage so that current does not exceed ratings (current is what strains supplies).  Further complicating matters, the total supply capacity is divided up 3 or 4 ways.  Likely the 5V rail is underloaded, but that doesn't help the 12V rail.   Sagging typically causes worse problems, like shutdowns.  It depends how close to the margin you are.  The most direct way to tell if sagging is causing what you see is to upgrade the power supply to the next bigger size.  You do have to buy a new supply, and it may not change anything, but then you would know what the problem isn't.

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Power Supply Max Load Capacity

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