- Apr 8, 2012
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Possibly one of the most common asked questions on this forum, how to select a power supply.
When choosing a power supply the following points need to be considered.
1. Watts
2. Amps on the +12v rail(s)
3. Quality of the PSU
4. What connections does your system require
5. 80+ efficiency ratings
1. Watts
Every component in the system will draw watts from the PSU. The graphics card tends to draw the most power. Many graphics cards will come with a recommended PSU wattage, and these rough estimates are a good number to go by. Here are some of the references I typically use to determine how many watts I should be looking for in my PSU
- http://www.realhardtechx.com/index_archivos/Page362.htm
This site not only tells you how many watts is recommended for your GPU, but what type of connections your PSU will need to have in order to connect to the GPU. This is discussed in more detail in section 4 below.
- http://forum-en.msi.com/faq/article/power-requirements-for-graphics-cards
another site that gives a rough estimate for how many watts you will need to power your system with a given graphics card. This site also gives you an estimate for how many amps your system will need to provide on it's +12v rail. This is discussed in more detail in section 2 below.
- http://www.extreme.outervision.com/psucalculatorlite.jsp
PSU calculators should be taken as rough estimates. This is the best calculator available. It also allows you to incorporate your overclock into the wattage calculation.
2. Amps on the +12v rail(s)
Watts is not the only important number you need to research when looking for a new PSU, especially if you are planning on powering a newer graphics card. The GPU will require a certain amount of amps on it's +12v rail. In order to determine how many amps you require, use this page here: http://forum-en.msi.com/faq/article/power-requirements-for-graphics-cards
Scroll down to your graphics card and write the number of amps required down. The rest of this section helps you determine if your PSU has at least this number of amps available.
There are a number of ways you can find out how many amps the PSU puts out.
a) you can look on the manufacturer's website. Here is an example for a popular PSU, the XFX 550W. http://xfxforce.com/en-us/products/xtr-series-full-modular/xtr-series-550w-psu-p1-550b-befx
If you click on the 'specs' tab on that page, you can scroll down and it tells you under 'load range' that this PSU puts out 45A on the +12v rail.
b) look at the PSU's sticker attached to it. Here is an example from the another XFX 550W PSU
You can see that it is providing 44A on the +12v rail.
Some PSUs will have multiple +12v rails, like this one for example.
This one has three +12v rails each with 18A. In order to determine how many amps this one has, you can not simply add 18A x3. You need to know how many watts the PSU has for its +12v rails in order to calculate how many amps. So in the sticker above, it tells us that there is 504W on the +12v rails. All you do is divide the +12v total watts by 12 volts. So in this case 504/12 = 42A total available on the +12v rails. Note that some PSU makers do not list the number of watts dedicated to the +12v rails on their stickers. This is a shady tactic to hide how many actual amps are available. A PSU from a reputable maker will always tell you how many watts are on the +12v rails.
c) you can also find out how many amps the PSU has by looking on this site's graph and finding your PSU. http://www.realhardtechx.com/index_archivos/Page541.htm
3. Quality of the PSU
Quality refers to the quality of the parts used in the construction of the PSU. The capacitors, the welding, transistors etc. The best way I have found to determine the quality of the PSU is simply by reading the reviews done by the key PSU testing sites. Jonnyguru.com and hardwaresecrets.com are two examples of sites that do extensive testing so we don't have to. Here is an amazing spreadsheet that has links to reviews done for hundreds of PSUs: http://www.realhardtechx.com/index_archivos/Page541.htm
Another useful link that can be used as a rough guideline is here. It breaks down the most common PSUs into a five tier system, tier one being high quality, tier 5 being dangerous to your system.
Do not go cheap on your power supply. When you use a low quality PSU and it dies, they have a tendency to take other parts out with them.
4. What connections does your system require
a) Graphics card connectors
Newer graphics cards require a special dedicated connection from the PSU directly to your graphics card. There are two types of connectors that are used, an 8 pin
or a 6 pin
Some PSUs provide what is called a 6+2 pin. These can be used as either an 8pin or a 6pin (by leaving the extra 2 hanging, unplugged)
Here is an example of a GPU requiring one 8pin and one 6pin connection
In order to determine what type of connectors you will need, you can research your graphics card here
http://www.realhardtechx.com/index_archivos/Page362.htm
Note that sometimes a manufacturer will make changes from the reference design. The page linked above is in general correct, but you need to go to the specific website for the GPU you are considering buying. For example the page above states that an R9 280X requires one 6pin and one 8pin, however the Sapphire Toxic version of the same GPU requires two 8pin connections. Always confirm for your specific model on its homepage.
You can find out what type and how many connectors your potential PSU has by either visiting the manufacturer's website, or by going here and finding your PSU in the charts http://www.realhardtechx.com/index_archivos/Page541.htm
b) Motherboard connectors
There are typically two connections that plug into your motherboard. The 24pin (some systems use a 20pin) and an 8pin cpu (some systems use a 4pin).
Most new power supplies come with a 20+4 pin. These are compatible with motherboards requiring either 20 or 24pin connectors.
Most new power supplies also come with a 4+4 cpu connector. These are compatible with motherboards requiring either a 8pin or a 4pin (by leaving the +4 hanging and not connected)
5. 80+ efficiency ratings
80plus is a certification for PSUs denoting their efficiency in energy use. 80plus rating means it is 80% efficient, Gold means 90% efficient at 50% load. In other words at 50% load an 80plus certified PSU will waste 20% energy as heat whereas a gold only wastes 10%.
In order for a platinum PSU to be worth while purchasing, the general consensus is that you'd need to be running your system at a high usage for years before you'd save enough money in your electricity bill to make it worth the investment. For the average home user, it's unnecessary. I wouldn't get a PSU that wasn't at least bronze though.
I hope this helps, and if you have any suggestions or comments please PM me and I'll make changes as needed.
When choosing a power supply the following points need to be considered.
1. Watts
2. Amps on the +12v rail(s)
3. Quality of the PSU
4. What connections does your system require
5. 80+ efficiency ratings
1. Watts
Every component in the system will draw watts from the PSU. The graphics card tends to draw the most power. Many graphics cards will come with a recommended PSU wattage, and these rough estimates are a good number to go by. Here are some of the references I typically use to determine how many watts I should be looking for in my PSU
- http://www.realhardtechx.com/index_archivos/Page362.htm
This site not only tells you how many watts is recommended for your GPU, but what type of connections your PSU will need to have in order to connect to the GPU. This is discussed in more detail in section 4 below.
- http://forum-en.msi.com/faq/article/power-requirements-for-graphics-cards
another site that gives a rough estimate for how many watts you will need to power your system with a given graphics card. This site also gives you an estimate for how many amps your system will need to provide on it's +12v rail. This is discussed in more detail in section 2 below.
- http://www.extreme.outervision.com/psucalculatorlite.jsp
PSU calculators should be taken as rough estimates. This is the best calculator available. It also allows you to incorporate your overclock into the wattage calculation.
2. Amps on the +12v rail(s)
Watts is not the only important number you need to research when looking for a new PSU, especially if you are planning on powering a newer graphics card. The GPU will require a certain amount of amps on it's +12v rail. In order to determine how many amps you require, use this page here: http://forum-en.msi.com/faq/article/power-requirements-for-graphics-cards
Scroll down to your graphics card and write the number of amps required down. The rest of this section helps you determine if your PSU has at least this number of amps available.
There are a number of ways you can find out how many amps the PSU puts out.
a) you can look on the manufacturer's website. Here is an example for a popular PSU, the XFX 550W. http://xfxforce.com/en-us/products/xtr-series-full-modular/xtr-series-550w-psu-p1-550b-befx
If you click on the 'specs' tab on that page, you can scroll down and it tells you under 'load range' that this PSU puts out 45A on the +12v rail.
b) look at the PSU's sticker attached to it. Here is an example from the another XFX 550W PSU
You can see that it is providing 44A on the +12v rail.
Some PSUs will have multiple +12v rails, like this one for example.
This one has three +12v rails each with 18A. In order to determine how many amps this one has, you can not simply add 18A x3. You need to know how many watts the PSU has for its +12v rails in order to calculate how many amps. So in the sticker above, it tells us that there is 504W on the +12v rails. All you do is divide the +12v total watts by 12 volts. So in this case 504/12 = 42A total available on the +12v rails. Note that some PSU makers do not list the number of watts dedicated to the +12v rails on their stickers. This is a shady tactic to hide how many actual amps are available. A PSU from a reputable maker will always tell you how many watts are on the +12v rails.
c) you can also find out how many amps the PSU has by looking on this site's graph and finding your PSU. http://www.realhardtechx.com/index_archivos/Page541.htm
3. Quality of the PSU
Quality refers to the quality of the parts used in the construction of the PSU. The capacitors, the welding, transistors etc. The best way I have found to determine the quality of the PSU is simply by reading the reviews done by the key PSU testing sites. Jonnyguru.com and hardwaresecrets.com are two examples of sites that do extensive testing so we don't have to. Here is an amazing spreadsheet that has links to reviews done for hundreds of PSUs: http://www.realhardtechx.com/index_archivos/Page541.htm
Another useful link that can be used as a rough guideline is here. It breaks down the most common PSUs into a five tier system, tier one being high quality, tier 5 being dangerous to your system.
Do not go cheap on your power supply. When you use a low quality PSU and it dies, they have a tendency to take other parts out with them.
4. What connections does your system require
a) Graphics card connectors
Newer graphics cards require a special dedicated connection from the PSU directly to your graphics card. There are two types of connectors that are used, an 8 pin
or a 6 pin
Some PSUs provide what is called a 6+2 pin. These can be used as either an 8pin or a 6pin (by leaving the extra 2 hanging, unplugged)
Here is an example of a GPU requiring one 8pin and one 6pin connection
In order to determine what type of connectors you will need, you can research your graphics card here
http://www.realhardtechx.com/index_archivos/Page362.htm
Note that sometimes a manufacturer will make changes from the reference design. The page linked above is in general correct, but you need to go to the specific website for the GPU you are considering buying. For example the page above states that an R9 280X requires one 6pin and one 8pin, however the Sapphire Toxic version of the same GPU requires two 8pin connections. Always confirm for your specific model on its homepage.
You can find out what type and how many connectors your potential PSU has by either visiting the manufacturer's website, or by going here and finding your PSU in the charts http://www.realhardtechx.com/index_archivos/Page541.htm
b) Motherboard connectors
There are typically two connections that plug into your motherboard. The 24pin (some systems use a 20pin) and an 8pin cpu (some systems use a 4pin).
Most new power supplies come with a 20+4 pin. These are compatible with motherboards requiring either 20 or 24pin connectors.
Most new power supplies also come with a 4+4 cpu connector. These are compatible with motherboards requiring either a 8pin or a 4pin (by leaving the +4 hanging and not connected)
5. 80+ efficiency ratings
80plus is a certification for PSUs denoting their efficiency in energy use. 80plus rating means it is 80% efficient, Gold means 90% efficient at 50% load. In other words at 50% load an 80plus certified PSU will waste 20% energy as heat whereas a gold only wastes 10%.
In order for a platinum PSU to be worth while purchasing, the general consensus is that you'd need to be running your system at a high usage for years before you'd save enough money in your electricity bill to make it worth the investment. For the average home user, it's unnecessary. I wouldn't get a PSU that wasn't at least bronze though.
I hope this helps, and if you have any suggestions or comments please PM me and I'll make changes as needed.
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