Note : This is not my personal article. Found some usefull info on the net so thought of sharing it with you guys particularly for noobs like me.
TE gurus are welcome to correct anything wrong on this article.
Thanks
Introduction
When building a new system, one of the most important components, is the one that is given least attention. The temptation is understandable, your new BE kit is costing you a serious chunk of money, and putting $150 towards a top of the range graphics card has to be more appealing than spending that money on a power supply. What's wrong with the $30 500W Power supply that came with the case? Quality, efficiency and power distribution is tested today like never before, and with the introduction of new technologies such as Nvidia's SLI, things are not getting any easier for your PSU.
It is therefore important to know what we are buying is suitable to the use we are putting it to, and to do this we need to understand the terms the manufacturers use, how they will make us think that a PSU is better than it actually is, and what our systems power requirement is.
Power Supply Specifications
Every power supply will have a label attached to it clearly defining its
specifications as outlined by the manufacturer.
What are Watts?
To understand this we need to understand volts, and amps:.
'Volts' are the potential difference of electrical energy between two points
'Amps' (or current) is the rate of flow of electrons
A 'Watt' is a measurement of 'volts' * 'amps', the ability to move electrons over 1amp a second
Henceforth, we will refer to 'Watts' as 'w', 'volts' as 'v', and 'amps' as 'a'
What are Rails?
A rail is the delivery system for power. On a modern PSU these rails are divided into +3.3v, +5v, +12v, -5v and -12v. The important ones for us to look at are the positive rails. These will be specified to deliver an amount of amps per rail. Seeing as w = v * a. This tells us the output in watts of the PSU.
EXAMPLE:
Watts =+3.3v rail*30a = 100w
Watts =+5v rail *30a = 150w
Watts =+12v rail*25a = 300w
Adding them together gives us a power output of 550w. There are caveats to this, which we will examine later.
The modern trend of components drawing off the +12v rail, has led to the introduction of multiple +12 rails, which are easier and cheaper to produce that bigger rails, but more importantly easier to keep stable and cool.
Power Supply Standards
The predominant standard at the moment, with the exception of a "small form factor" PC, is ATX 12v. This features a 20-pin connector to the motherboard, and in addition to the ATX standard, a 4-pin +12v connector. This is due to modern CPU using the +12v rail, whereas a Pentium III operated across +5v. The standard also sets out standards for power fluctuation, which must not exceed (+ or -)5% per positive rail. ATX 12v version 2, released in Feb 2003, requires SATA connectors. We are now at version 2.01. A history of versions can be found here.
An additional standard exists - ATX EPS 12v - for server and workstation. This specifies a 24-pin main connector and an 8-pin +12v connector for the motherboard. With the move to socket 775, Intel has updated the main motherboard connector to 24-pin.
A new standard on the horizon is BTX - Balanced Technology Extended - which amongst other things will address issues of heat and airflow by placing components in a 3D plane rather than the existing 2D, there are other advantages, but they are beyond the scope of this article. Some power supplies are already on the market claiming to be BTX ready, and may be a way to future proof your investment.
Cables
Molex Connector - used for Hard Drives,
Fans and Add on Cards
Main motherboard 20
pin connector
Extra 12v connector
for CPU
SATA power connector pictured
next to molex, for use
with SATA devices
PCI-e power connector for PCI-e graphics cards - supplies
additional power to the more power hungry PCI-e graphics cards
Please note that only a few existing PSUs currently support a 6-pin PCI-e connector, so add in the cost of a converter or two when specifying the PSU.
Modular concepts are now being released, where only the cabling you require is connected to the PSU. This approach makes for a much neater looking case, and helps to improve airflow. Some manufacturers supply the cables braided, which look very neat, especially with a case window. Alternatively, some on-line retailers will sleeve you PSU with whatever you choose. Just remember it should always be function over form.
Negative Rails
You will notice that the negative rails on your PSU are very week when compared to the positive rails. This is due to them being there more or less for legacy reasons. Although a 20 pin ATX motherboard connector has pin 12 -12v and pin 18 -5v, they're generally not used by the motherboard. They are there for:
ISA cards
Serial port or LAN
Older FDD
Hold Up Time
This is the amount of time in milliseconds that a PSU can hold output at the correct voltages after a loss of input power. This allows your PC to carry on running, despite a brief interruption in AC power, the higher the figure the better. The ATX specification requires a minimum of 17ms.
What is PFC?
PFC is Power Factor Correction. Power Factor is a measurement of how effectively power is being used, and is expressed as a number between 0 and 1, with the high number meaning that power is being used effectively.
Two types of load are placed on the PSU, resistive; power converted into heat, light motion, i.e. working power. Inductive; sustaining an electric field in a transformer or motor, also called reactive power. Together, working and reactive power make up apparent power. Power factor is a measurement of the ratio between working and reactive power, or working power/reactive power. The ideal is for the two to be the same, i.e. a PF of 1.
There are two types of PFC, passive and active. Passive uses a capacitive filter on the AC input to maintain the inductive load, without an addition of power. Active uses a circuit to match the resistive and inductive loads.
Every PSU sold in the Europe is now required to have PFC, although when I bought mine, I knew nothing about this, and the dealer supplied me with a US model, complete with 110V settings. If you have a non PFC PSU, always make sure that the voltage is set correctly for your region as damage will result if not. This is another benefit of PFC; it automatically senses and adjusts to the input voltage.
The common misconception is, that PFC will lead to lower electricity bills. This is not true unless you are a commercial user who is charged by Volt Amps and PFC, rather than KW/hr.
The benefits are to the environment, keeping power clean, reducing harmonics. . For a more detailed look at PFC, see Adrian's Rojak Pot. Please note if you have a room full of PCs, e.g. a folding farm a high PFC (lowering current required) will allow you to connect more PCs, to a single circuit.
Peak Vs Continuous Power
Continuous power is what the PSU is capable of delivering all the time. Peak is for short bursts - e.g. 60 seconds. To run a PSU above its continuous power rating for an extended period of time, will result in damage, so make sure when specifying to get the correct figure.
Noise Level
This will be measured in db. It is very important to match this to the environment you will be using your PC in. In a noisy office environment, 30db may be fine. In a living room of your house, 30db will cause arguments between you and your partner. As a general rule, a PSU with a 120mm fan should be quieter than one with a higher pitched 92mm or 80mm fan. You need to be looking for fan speed to be adjustable according to PSU load/temperature. As a PSU works harder, it will heat up, and cause the fans to work harder. As an example, a PSU may work at 20db when at up to 70% capacity, going up to 32db as it nears 95%. Take this into account when working out which power supply you want. If you need 300W, and to get it you need to operate the PSU at 85%, then you may be better off going for the next model, that can supply it at 70%, and all things being equal, do so more quietly.
As for totally a silent PSU, with passive cooling - based on all reviews I have read, I cannot recommend them for a BE type system, on a cost performance basis. They are very expensive, so much so that you could get a real high end active cooling solution, and under-stress it as described above. A passive PSU is extreme in its own way, and thus only recommended if you live in a library.
Water-cooling is another way to silence your PSU. There are a few articles published on the web on 'how to' water-cool a PSU. For the sake of interest, I will include a couple of links at the end of this guide. However this is not something I can recommend, due to the charge that a PSU will hold for days. It is dangerous, and good luck to you if you embark on such a project!
Adjustable Pots
These allow you to adjust the voltage to each rail, for instance if the +12v rail is reading low at +11.5v, this could cause some system instability. Remember, the ATX standard allows for a variation of +-5%. Voltage can be adjusted back up to +12v by use of the 'pots', or potentiometers. This is quite an advanced feature, so you need to make sure you know what you are doing, and I would suggest further reading, if you are an enthusiast.
SO WHAT PSU IS SUITABLE FOR MY NEEDS?
Inappropriate Power Supply
In our look at PSU specifications, we noted that the current standard is ATX 12v. Anything that is of an older specification should be avoided with a modern system. A complete ATX 12v Power Supply Design Guide linked here is available at formfactors.org.
Working Out Power Requirement
When doing this, I work on the principle that it is better to be safe than sorry, so start by working out the maximum possible power draw. Note, it is unlikely that you will use all your hard drives, all your optical drives and stress both CPU and graphics card to the maximum a one time, but it is possible. If you feel confident that you will not be stressing your system 100% for any period of time then you can follow a different method of calculation, based on burst power usage, and using a factor (usually 80%), to allow for your personal PC usage. However - this is BE! So I'm going to assume we are all going to stress our system to the max!
Worked Example
My next build will be an Nvidia Nforce 4 based dual SLI system, so I will use that for a power hungry example.
As you can see from the table, it's not as simple as saying a graphics card uses the 12v rail, as it uses all 3. Not only this but it takes its power from both the AGP slot and a molex connector.
Some of these figures are very difficult to track down, and where that is the case, I've used estimates based on other 'known' components.
From this table, we can see that theoretically run our PC from a 360w PSU. Some companies may do that, but there is no way we are going to. We need to allow a safe working margin, and to some extent this is a personal thing, so we'll look at a couple of examples.
Example One - No overclocking, stress margin, upgrade headroom
The Stress Margin is there for us not to run the PSU at 100%. This will vary depending on how safe you want to be, and if you want to allow for quieter operation of the PSU. With the former being the case, I would personally allow 10%. With the later, you may be over-specifying by up to 20%, to allow the fan not to spin up to full - and this will allow you plenty of room to experiment when the other half is out of the house.
Upgrade headroom is exactly what it sounds like. If in the future you want to add extra case fans, neon's, or even upgrade the graphics card, you're going to be annoyed have to part with your quality expensive PSU just because you didn't over-specify in the first place. For this I'm going to allow a personal factor of 10%. Yours may and will vary.
This gives us a safety margin of 20% (assuming noise isn't an issue). So out power supply would need to be 360w * 120% = 432W. We also need to apply this margin to the individual rails. Any modern PSU that can supply the required voltage to the +12v rails, is going to be able to supply the voltage required to +3,3v and +5v rails, so I will just apply the factor to the +12v rail. Therefore +12v requirement = 18.84*145% = 23.55a.
SO WHAT PSU IS SUITABLE FOR MY NEEDS? .. Continued
Example Two - Allowing for a Heavy Overclock
When this is the case we will be over volting components, and operating them beyond their rated frequencies. For this I'm going to allow 25%. This may sound like a lot, but the last thing you want to happen is that your overclock fails because the power supply won't cope. Taken with our stress margin - 10%, and our upgrade margin - 10%, this leaves us a personal safe working margin of 45%! So power supply would need to be 360w * 145% = 522w! The +12v rail will now be 18.84 * 145% = 27.3a.
Remember this is my personal preference, and other opinions vary.
Once we have found a PSU to satisfy both the wattage and the +12v rail demand, we need to make sure that the +12v rail is properly set up to deal with our components.
For instance, a Pentium 4 3.4EE requires almost 11a on the 12v rail to itself. If you attached it to a PSU with 2 +12v rails, giving 14a each, the requirement of total amps is met, but you have very little headroom to overclock.
WHAT TO LOOK OUT FOR - or - How Do Some Manufacturers Cheat?
Now we know a lot of the specifications that a manufacturer uses, we can look into making sure we get what we think we are getting.
Example One:
Output
+3.3v rail = 30a
+5v rail = 40a
+12v rail = 34a
Continuous power = 510W max
Peak power = 650W max
Remembering that w = v * a, this tells us that the PSU is able to output:
100w on +3.3v rail
200w on +5v rail
408w on +12v rail
Example Two:
Output
+3.3v rail = 30a
+5v rail = 40a
+12v rail = 30a
Total power = 660W max
Measurements taken at 40F
Remembering that w = v * a, this tells us that the PSU is able to output:
100w on +3.3v rail
200w on +5v rail
360w on +12v rail
"Always read specifications carefully, and take nothing for granted, for example Antec produces three different 480w PSUs. The Antec truepower has a 28a +12v rail, the trueblue only a 22a +12v rail. If you don't do your homework properly you will get caught out."
Stop Right There!
Straight off, we can see that the manufacturer has quoted the max figure for the PSU - and that max figure, is the total of all rails. Wait! The figures are all quoted at 40F/-4C! This PSU has been tested in a cold box, so unless you are going to do the same, you're going to have trouble the first time you load it up, as the normal operating temperature is probably going to be nearer 100F/38C. At this temperature I would be surprised if you could get a continuous output of 300W. The thing here is the manufacturer isn't lying. The figures as presented are correct. It is up to us the customers, to interpret them. Don't expect it to be as easy as this example. The manufacturer isn't going to tell you they have taken measurements at low temperatures.
As a general rule of thumb, if a PSU looks too cheap to be true, it is exactly that. This is why you should stick to well known manufacturers. Seek a recommendation from a friend, read them specifications very carefully, read as many reviews as possible, and ask at TE.
TE gurus are welcome to correct anything wrong on this article.
Thanks
Introduction
When building a new system, one of the most important components, is the one that is given least attention. The temptation is understandable, your new BE kit is costing you a serious chunk of money, and putting $150 towards a top of the range graphics card has to be more appealing than spending that money on a power supply. What's wrong with the $30 500W Power supply that came with the case? Quality, efficiency and power distribution is tested today like never before, and with the introduction of new technologies such as Nvidia's SLI, things are not getting any easier for your PSU.
It is therefore important to know what we are buying is suitable to the use we are putting it to, and to do this we need to understand the terms the manufacturers use, how they will make us think that a PSU is better than it actually is, and what our systems power requirement is.
Power Supply Specifications
Every power supply will have a label attached to it clearly defining its
specifications as outlined by the manufacturer.
What are Watts?
To understand this we need to understand volts, and amps:.
'Volts' are the potential difference of electrical energy between two points
'Amps' (or current) is the rate of flow of electrons
A 'Watt' is a measurement of 'volts' * 'amps', the ability to move electrons over 1amp a second
Henceforth, we will refer to 'Watts' as 'w', 'volts' as 'v', and 'amps' as 'a'
What are Rails?
A rail is the delivery system for power. On a modern PSU these rails are divided into +3.3v, +5v, +12v, -5v and -12v. The important ones for us to look at are the positive rails. These will be specified to deliver an amount of amps per rail. Seeing as w = v * a. This tells us the output in watts of the PSU.
EXAMPLE:
Watts =+3.3v rail*30a = 100w
Watts =+5v rail *30a = 150w
Watts =+12v rail*25a = 300w
Adding them together gives us a power output of 550w. There are caveats to this, which we will examine later.
The modern trend of components drawing off the +12v rail, has led to the introduction of multiple +12 rails, which are easier and cheaper to produce that bigger rails, but more importantly easier to keep stable and cool.
Power Supply Standards
The predominant standard at the moment, with the exception of a "small form factor" PC, is ATX 12v. This features a 20-pin connector to the motherboard, and in addition to the ATX standard, a 4-pin +12v connector. This is due to modern CPU using the +12v rail, whereas a Pentium III operated across +5v. The standard also sets out standards for power fluctuation, which must not exceed (+ or -)5% per positive rail. ATX 12v version 2, released in Feb 2003, requires SATA connectors. We are now at version 2.01. A history of versions can be found here.
An additional standard exists - ATX EPS 12v - for server and workstation. This specifies a 24-pin main connector and an 8-pin +12v connector for the motherboard. With the move to socket 775, Intel has updated the main motherboard connector to 24-pin.
A new standard on the horizon is BTX - Balanced Technology Extended - which amongst other things will address issues of heat and airflow by placing components in a 3D plane rather than the existing 2D, there are other advantages, but they are beyond the scope of this article. Some power supplies are already on the market claiming to be BTX ready, and may be a way to future proof your investment.
Cables
Molex Connector - used for Hard Drives,
Fans and Add on Cards
Main motherboard 20
pin connector
Extra 12v connector
for CPU
SATA power connector pictured
next to molex, for use
with SATA devices
PCI-e power connector for PCI-e graphics cards - supplies
additional power to the more power hungry PCI-e graphics cards
Please note that only a few existing PSUs currently support a 6-pin PCI-e connector, so add in the cost of a converter or two when specifying the PSU.
Modular concepts are now being released, where only the cabling you require is connected to the PSU. This approach makes for a much neater looking case, and helps to improve airflow. Some manufacturers supply the cables braided, which look very neat, especially with a case window. Alternatively, some on-line retailers will sleeve you PSU with whatever you choose. Just remember it should always be function over form.
Negative Rails
You will notice that the negative rails on your PSU are very week when compared to the positive rails. This is due to them being there more or less for legacy reasons. Although a 20 pin ATX motherboard connector has pin 12 -12v and pin 18 -5v, they're generally not used by the motherboard. They are there for:
ISA cards
Serial port or LAN
Older FDD
Hold Up Time
This is the amount of time in milliseconds that a PSU can hold output at the correct voltages after a loss of input power. This allows your PC to carry on running, despite a brief interruption in AC power, the higher the figure the better. The ATX specification requires a minimum of 17ms.
What is PFC?
PFC is Power Factor Correction. Power Factor is a measurement of how effectively power is being used, and is expressed as a number between 0 and 1, with the high number meaning that power is being used effectively.
Two types of load are placed on the PSU, resistive; power converted into heat, light motion, i.e. working power. Inductive; sustaining an electric field in a transformer or motor, also called reactive power. Together, working and reactive power make up apparent power. Power factor is a measurement of the ratio between working and reactive power, or working power/reactive power. The ideal is for the two to be the same, i.e. a PF of 1.
There are two types of PFC, passive and active. Passive uses a capacitive filter on the AC input to maintain the inductive load, without an addition of power. Active uses a circuit to match the resistive and inductive loads.
Every PSU sold in the Europe is now required to have PFC, although when I bought mine, I knew nothing about this, and the dealer supplied me with a US model, complete with 110V settings. If you have a non PFC PSU, always make sure that the voltage is set correctly for your region as damage will result if not. This is another benefit of PFC; it automatically senses and adjusts to the input voltage.
The common misconception is, that PFC will lead to lower electricity bills. This is not true unless you are a commercial user who is charged by Volt Amps and PFC, rather than KW/hr.
The benefits are to the environment, keeping power clean, reducing harmonics. . For a more detailed look at PFC, see Adrian's Rojak Pot. Please note if you have a room full of PCs, e.g. a folding farm a high PFC (lowering current required) will allow you to connect more PCs, to a single circuit.
Peak Vs Continuous Power
Continuous power is what the PSU is capable of delivering all the time. Peak is for short bursts - e.g. 60 seconds. To run a PSU above its continuous power rating for an extended period of time, will result in damage, so make sure when specifying to get the correct figure.
Noise Level
This will be measured in db. It is very important to match this to the environment you will be using your PC in. In a noisy office environment, 30db may be fine. In a living room of your house, 30db will cause arguments between you and your partner. As a general rule, a PSU with a 120mm fan should be quieter than one with a higher pitched 92mm or 80mm fan. You need to be looking for fan speed to be adjustable according to PSU load/temperature. As a PSU works harder, it will heat up, and cause the fans to work harder. As an example, a PSU may work at 20db when at up to 70% capacity, going up to 32db as it nears 95%. Take this into account when working out which power supply you want. If you need 300W, and to get it you need to operate the PSU at 85%, then you may be better off going for the next model, that can supply it at 70%, and all things being equal, do so more quietly.
As for totally a silent PSU, with passive cooling - based on all reviews I have read, I cannot recommend them for a BE type system, on a cost performance basis. They are very expensive, so much so that you could get a real high end active cooling solution, and under-stress it as described above. A passive PSU is extreme in its own way, and thus only recommended if you live in a library.
Water-cooling is another way to silence your PSU. There are a few articles published on the web on 'how to' water-cool a PSU. For the sake of interest, I will include a couple of links at the end of this guide. However this is not something I can recommend, due to the charge that a PSU will hold for days. It is dangerous, and good luck to you if you embark on such a project!
Adjustable Pots
These allow you to adjust the voltage to each rail, for instance if the +12v rail is reading low at +11.5v, this could cause some system instability. Remember, the ATX standard allows for a variation of +-5%. Voltage can be adjusted back up to +12v by use of the 'pots', or potentiometers. This is quite an advanced feature, so you need to make sure you know what you are doing, and I would suggest further reading, if you are an enthusiast.
SO WHAT PSU IS SUITABLE FOR MY NEEDS?
Inappropriate Power Supply
In our look at PSU specifications, we noted that the current standard is ATX 12v. Anything that is of an older specification should be avoided with a modern system. A complete ATX 12v Power Supply Design Guide linked here is available at formfactors.org.
Working Out Power Requirement
When doing this, I work on the principle that it is better to be safe than sorry, so start by working out the maximum possible power draw. Note, it is unlikely that you will use all your hard drives, all your optical drives and stress both CPU and graphics card to the maximum a one time, but it is possible. If you feel confident that you will not be stressing your system 100% for any period of time then you can follow a different method of calculation, based on burst power usage, and using a factor (usually 80%), to allow for your personal PC usage. However - this is BE! So I'm going to assume we are all going to stress our system to the max!
Worked Example
My next build will be an Nvidia Nforce 4 based dual SLI system, so I will use that for a power hungry example.
As you can see from the table, it's not as simple as saying a graphics card uses the 12v rail, as it uses all 3. Not only this but it takes its power from both the AGP slot and a molex connector.
Some of these figures are very difficult to track down, and where that is the case, I've used estimates based on other 'known' components.
From this table, we can see that theoretically run our PC from a 360w PSU. Some companies may do that, but there is no way we are going to. We need to allow a safe working margin, and to some extent this is a personal thing, so we'll look at a couple of examples.
Example One - No overclocking, stress margin, upgrade headroom
The Stress Margin is there for us not to run the PSU at 100%. This will vary depending on how safe you want to be, and if you want to allow for quieter operation of the PSU. With the former being the case, I would personally allow 10%. With the later, you may be over-specifying by up to 20%, to allow the fan not to spin up to full - and this will allow you plenty of room to experiment when the other half is out of the house.
Upgrade headroom is exactly what it sounds like. If in the future you want to add extra case fans, neon's, or even upgrade the graphics card, you're going to be annoyed have to part with your quality expensive PSU just because you didn't over-specify in the first place. For this I'm going to allow a personal factor of 10%. Yours may and will vary.
This gives us a safety margin of 20% (assuming noise isn't an issue). So out power supply would need to be 360w * 120% = 432W. We also need to apply this margin to the individual rails. Any modern PSU that can supply the required voltage to the +12v rails, is going to be able to supply the voltage required to +3,3v and +5v rails, so I will just apply the factor to the +12v rail. Therefore +12v requirement = 18.84*145% = 23.55a.
SO WHAT PSU IS SUITABLE FOR MY NEEDS? .. Continued
Example Two - Allowing for a Heavy Overclock
When this is the case we will be over volting components, and operating them beyond their rated frequencies. For this I'm going to allow 25%. This may sound like a lot, but the last thing you want to happen is that your overclock fails because the power supply won't cope. Taken with our stress margin - 10%, and our upgrade margin - 10%, this leaves us a personal safe working margin of 45%! So power supply would need to be 360w * 145% = 522w! The +12v rail will now be 18.84 * 145% = 27.3a.
Remember this is my personal preference, and other opinions vary.
Once we have found a PSU to satisfy both the wattage and the +12v rail demand, we need to make sure that the +12v rail is properly set up to deal with our components.
For instance, a Pentium 4 3.4EE requires almost 11a on the 12v rail to itself. If you attached it to a PSU with 2 +12v rails, giving 14a each, the requirement of total amps is met, but you have very little headroom to overclock.
WHAT TO LOOK OUT FOR - or - How Do Some Manufacturers Cheat?
Now we know a lot of the specifications that a manufacturer uses, we can look into making sure we get what we think we are getting.
Example One:
Output
+3.3v rail = 30a
+5v rail = 40a
+12v rail = 34a
Continuous power = 510W max
Peak power = 650W max
Remembering that w = v * a, this tells us that the PSU is able to output:
100w on +3.3v rail
200w on +5v rail
408w on +12v rail
Example Two:
Output
+3.3v rail = 30a
+5v rail = 40a
+12v rail = 30a
Total power = 660W max
Measurements taken at 40F
Remembering that w = v * a, this tells us that the PSU is able to output:
100w on +3.3v rail
200w on +5v rail
360w on +12v rail
"Always read specifications carefully, and take nothing for granted, for example Antec produces three different 480w PSUs. The Antec truepower has a 28a +12v rail, the trueblue only a 22a +12v rail. If you don't do your homework properly you will get caught out."
Stop Right There!
Straight off, we can see that the manufacturer has quoted the max figure for the PSU - and that max figure, is the total of all rails. Wait! The figures are all quoted at 40F/-4C! This PSU has been tested in a cold box, so unless you are going to do the same, you're going to have trouble the first time you load it up, as the normal operating temperature is probably going to be nearer 100F/38C. At this temperature I would be surprised if you could get a continuous output of 300W. The thing here is the manufacturer isn't lying. The figures as presented are correct. It is up to us the customers, to interpret them. Don't expect it to be as easy as this example. The manufacturer isn't going to tell you they have taken measurements at low temperatures.
As a general rule of thumb, if a PSU looks too cheap to be true, it is exactly that. This is why you should stick to well known manufacturers. Seek a recommendation from a friend, read them specifications very carefully, read as many reviews as possible, and ask at TE.
