Why my psu max load (wattage) shows different than i have calculate on the label

[jerry_96]

HELLO,

I Want to know.

does my psu 450w can handle this agp radeon geforce 7800gs and radeon gd 3850 ? (do not wish to change psu or mobo).


Secondly,why my psu label says 450w but when i calculated the amps and volt i only got 243W.My psu have 12v+ 10amps.

my pc info is

xp sp3
mobo u8668 biostar
ram ddr 1 gb
agp 4x slot
intel celeron d 2.26ghz

 

[Toxxyc]

PSU manufacturers lie to make their PSU's look more attractive and powerful than it really is.

 

[lilotimz]

Replace it. Get something like an Antec EA 380D or Corsair CX 430 V2. Both have 28+ Amps on the 12v rail and is of good build quality while affordable (even more so when on sale).

Second, PSU companies are required to have that table showing voltage etc but there is no specific thing that says they have to say. So crappy companies put their MAX voltage while good companies put CONTINUOUS voltage and the certifications and tests that say they're good quality. BIG distinction between those two. Not to mention the wattage doesn't have to come from the 12v line but can also be from the 5v and 3.5v.

 

[jerry_96]

does my psu 450w can handle this agp radeon geforce 7800gs and radeon hd 3850 ? (do not wish to change psu or mobo).

 

[jerry_96]

thanks,how about the first question i'm asking?

 

[jsc]

Depending on the model, a 7800 needs about 4 - 6 amps. A 3850 needs about 5 - 7 amps.

Your PSU is an old design with relatively heavy 3.3 and 5 volt rails and a pretty light 12 volt rail. A modern 450 watt PSU should produce at least 30 amps.

You will need a new PSU.

Replace it. Get something like an Antec EA 380D or Corsair CX 430 V2. Both have 28+ Amps on the 12v rail and is of good build quality while affordable (even more so when on sale).

I would prefer the Antec over the Corsair CX430.

Second, PSU companies are required to have that table showing voltage etc but there is no specific thing that says they have to say. So crappy companies put their MAX [strike]voltage[/strike] current while good companies put CONTINUOUS [strike]voltage[/strike] current and the certifications and tests that say they're good quality. BIG distinction between those two. Not to mention the wattage doesn't have to come from the 12v line but can also be from the 5v and 3.5v.

In modern systems, the CPU and GPU (the two things that consume the most power) are powered from the 12 volt lines.

 

[RetiredChief]

(1) You can not calculate the power drawn by a given system as you can not measure the individual currents that are drawn on the given system.

(2) If you are calculating the max wattage based on the label then this will NOT be correct. manuf list the max wattage for a given rail with the other rails at a minimum value. Therefore when you add up the wattages they will always be equal to, and in MOST cases exceed the max rated value. There few PSUs can deliever the max rated on all rails at the same time.

Quote: Secondly,why my psu label says 450w but when i calculated the amps and volt i only got 243W.My psu have 12v+ 10amps End quote. I have never seen a PSU where the sum of all rails at max rating is LESS than max rated for the PSU.

 

[jerry_96]

so the label that are shows the amps and volt are not correct?

 

[RetiredChief]

The labels are correct; But often misleading.
(1) The max listed for an individual Rail is often based on little to no power on the other Rails. This is the reason that if you calculate and added up the max for the indivdual rails they will exceed the overall max listed for the PSU.

(2) Often the max ratings are determined at a given temperature. If the manuf keeps the PSU at 20 C and determines the ratings then when a user runs the PSU in a system and the temperature goes up there most will see a drop in ability to provide power. Some of the poorer PSUs will see the max capability drop by 30->50%. IE a 700 PSU only able to supply 350 -> 500 Watts in the REAL world. "Quality" PSUs provide the ratings at NORMAL operating conditions.

This is the reason you should always look for independent reviews of a PSU you have, or are planning on buying.

 

[jerry_96]

Best answer selected by jerry_96.