There is no correct answer. It depends on your own specific pc case ventilation, airflow, and cooling configuration.
Many years ago the original atx specifications proposed by Intel called for the power supply to be installed near the cpu because the power supply had a fan which could exhaust hot air out of the case. That was a long time ago when cpu's did not have heatsinks and cases did not have fans. A lot has changed since then.
Quite a few modern gaming cases are designed for power supplies mounted in the bottom rear of the case.
One idea behind the design concept is that a case with a heavy power supply mounted in the bottom is less likely to tip over than a case with a psu mounted at the top.
Another idea behind the design concept is that by moving the psu to the bottom of the case there is room for an exhaust fan at the rear of the top panel which could help to exhaust warm air.
A third idea behind the design concept is that the psu fan which pulls in warm interior air and exhausts it out the rear panel can help cool the video card which is installed just above it.
I have a Lancool Dragon Lord case for my new system. I have a Coolermaster HAF 932 for my emergency backup system. I've also had other cases with bottom mounted cases. You have two choices with a bottom mounted power supply.
First, if your case has sufficient ventilation, airflow, and cooling, then you could mount the psu fan side down so it draws in cool air from the opening in the bottom of the case and exhausts warm air out the rear of the case. The idea is that by drawing in its own supply of cool air rather than drawing in warm case air the power supply will be easier to cool and will last longer. However, do not place the case on a carpeted floor, especially deep pile carpeting. The carpet fibers will block air flow. A few years ago I made my own pc stand with casters.
Second, you could install the power supply with the psu fan on top. The concept is that the psu would help cool the interior of the case by drawing in warm interior air and exhausting it out the rear of the case.
The ventilation, airflow, and cooling in my new Dragon Lord case and my older HAF 932 are excellent so I mounted my power supplies with the psu fans on the bottom.
However that being said you really have to watch out because depending on the power source, the 4/8 pin connector for the cpu will be too short. Which isn't that big of a problem, you can always buy an extension for a couple dollars.
MechaNintendoMaste - Good cable management on the part of the user will eliminate dangling cables.
JordoR - Yes, the 4/8 pin power cable really does depend on the power supply. My back-up pc is in a Coolermaster HAF 932 with bottom mounted Corsair power supply. When I bought the case I also pruchased an 8 pin cable extension. I was thinking the psu cable would not be able to reach the connection on the motherboard. Much to my surprise I was able to run the cable up the back of the motherboard tray and plug it in without the cable extension.
hey i want to know what its better and why have a bottom mounted power supply or have a top mounted power suply
Funny... I was looking for the exact opposite (see here). I've been looking for a good case which has Power Supply at the top. They are hard to find these days.
Advantages of top-mounted power supply:
* Hot air rises so the area at the top of the case gets warmer and warmer which leads to higher ambient temperature inside the case. This heat at the top needs to be throw out. Power supply is the best component to accomplish this task since it has a fan underneath it sucking the heat into itself and simultaneously another fan throwing the heat out of the case. Most of this heat comes from the CPU and Graphics card which are directly below it.
Disadvantages of bottom-mounted power supply:
* Need to add a fan near the top of the case to remove the accumulating heat at the top. Clearly one fan sucking air out is not better than two fans that were present had the power supply been there. Adding extra fans will also generate more noise to the system. Why add extra fans when power supply could have done this job without extra fans.
* Cable management can be difficult.
* Heat generated from the power supply rises and heats up the graphics card.
* Bottom-mounted power supplies have the fan facing down towards the floor. This will cause them to suck in dust and debris from the carpet and the surrounding, which will eventually clog the power supply up and thus generate more heat and lower efficiency.
* There isn't enough room at the bottom of the case (low ventilation) for the power supply to suck the air efficiently compared to the power supply being at the top. Air has to be sucked through tight spaces around the case into the power supply. This will also increase the suction of dust and debris from the outside due to increased pressure.
You might say that you could turn the power supply upside down at the bottom of the case. But some cases don't allow you to turn the power supply upside down due to the fix screws on the case. Also, turning it upside down wouldn't be as effect as throwing warm air out of the case had the power supply been mounted at the top because that's where the warm air is, which needs to be thrown out.
I would never buy a case with the power supply on the bottom. So why are most cases these days with power supplies at the bottom? Simple: customer demand - and thus sales.
Tomshardware had documented the disadvantages of a bottom-mounted power supply in an article here: