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In this guide I will explain what exactly to look for in a PSU. Most people by far overestimate the power their system is going to consume. Computers are a lot more frugal than you think.
Wattage:
Whilst it is for the most part considered one of the most important factors, it is in fact not. A PSU may be rated at 750W, but if it's supplying half that power across the 3.3V and 5V rails, it's not much use. You should be looking for how much power the PSU is providing across the 12V rail(s)
Wattage = Voltage x Amperage. For example, if there is 30A on the 12V rail: 30 x 12 = 360W. The PSU is supplying 360W across the 12V rail(s).
Be careful when you see a high wattage PSU at a strangely low price. There is a reason the price is so low. Poor quality components? Ridiculously good testing conditions? (PC Power & Cooling for example test all their PSUs at 50c before rating them) Who knows how long it could last before it "blows up"? Is there over voltage protection so it doesn't take your whole system with it? I hope I got my message across, quality over quantity. Besides, we already know that wattage is not the most important factor, so don't get reeled in by that 1000W at $50.
12V rail(s) and wattage distribution:
More is better? Less is better? The truth is... it doesn't really matter. Whilst some people claim that since each rail of a multi rail PSU can provide less power and therefore there is a possibility of one becoming overloaded, whereas this would never happen on a single rail PSU. Now, not every component is going to be pulling power from the same rail, is it? So it is almost impossible to overload one of the rails. You are not the guy who is overclocking a Phenom TWKR to 7GHz under liquid nitrogen and running a couple of power hungry graphics cards at the same time. There, one of the rails may become overloaded. But for 99.9% of users, it's never going to happen, so don't let it influence your purchase too much.
Just look out for one thing, many brands rate the rails of their PSUs at the maximum current one rail carry. For example, this Xigmatek has four rails each rated at 18A, it's a 750W PSU. (18 x 4) x 12 = 864W. Obviously, that's impossible, not all rails can carry 18A at the same time. So try to find out from the manufacturer exactly what amperage the PSU can output along the 12V rails if your see ten 20A rails on a 400W PSU (Exaggeration)
Efficiency:
Efficiency is not going to affect the performance of the PSU. The power output of the PSU is lower than the power it is drawing from the wall. The higher the efficiency rating, the less power is wasted and less heat is produced. Although, it could save you some money in the long run. And it could give you an idea of the quality of PSUs.
Brand:
Although there are countless brands of PSUs out there, very few of them actually manufacture their own. Big brands like Corsair, Antec, Hiper, Thermaltake... are simply rebranders. So think about that next time you pay more for a Corsair VX than a Xigmatek who are both manufactured by CWT.
Here is a link to the different OEMs for different brands.
How much power do I need?
The wattage recommended nowadays for some systems is ridiculous. Your water cooled, overclocked GTX 295 SLi system may actually be using a lot less power than you think. No, you don't need that 1.5KW $750 PSU.
Take at look at these graphs, they show the total system power under full load for a number of systems. A lot lower than you thought, huh?
Office Computer:
* CPU: Intel Pentium Dual-Core E2220 (2.4GHz)
* CPU cooler: GlacialTech Igloo 5063 Silent (E) PP
* Fan: GlacialTech SilentBlade II GT9225-HDLA1
* Mainboard: Gigabyte GA-73PVM-S2 (nForce 7100 chipset)
* System memory: 1GB Samsung (PC6400, 800MHz, CL6)
* Hard disk drive: 160GB Hitachi Deskstar 7K1000.B HDT721016SLA380
* Optical drive: DVD±RW Optiarc AD-7201S
* Card-reader: Sony MRW620
* System case: IN-WIN EMR-018 (350W)
Home PC:
* Processor: AMD Athlon 64 X2 5000+ (2.60GHz)
* CPU cooler: TITAN DC-K8M925B/R
* GlacialTech SilentBlade II GT9225-HDLA1
* ASUS M3A78 (AMD 770 chipset)
* System memory: 2x1GB Samsung (PC6400, 800MHz, CL6)
* Hard disk drive: 250GB Seagate Barracuda 7200.10 ST3250410AS
* Graphics card: 512MB Sapphire Radeon HD 4650
* Optical drive: DVD±RW Optiarc AD-7201S
* System case: IN-WIN EAR-003 (400W)
File Server:
* Processor: AMD Athlon 64 X2 5000+ (2.60GHz)
* CPU cooler: TITAN DC-K8M925B/R
* GlacialTech SilentBlade II GT9225-HDLA1
* ASUS M3A78 (AMD 770 chipset)
* System memory: 2x1GB Samsung (PC6400, 800MHz, CL6)
* Hard disk drive: 250GB Seagate Barracuda 7200.10 ST3250410AS
* Graphics card: 512MB Sapphire Radeon HD 4650
* Optical drive: DVD±RW Optiarc AD-7201S
* System case: IN-WIN EAR-003 (400W)
* Hard driives: 3 x 74GB Western Digital Raptor WD740GD
Gaming Computer:
* Processor: Intel Core 2 Duo E8600 (3.33GHz)
* CPU cooler: GlacialTech Igloo 5063 PWM (E) PP
* Mainboard: ASUS P5Q (iP45 chipset)
* System memory: 2x2GB DDR2 SDRAM Kingston ValueRAM (PC6400, 800MHz, CL6)
* Hard disk drive: 500GB Seagate Barracuda 7200.12
* Graphics card: PCI-E 512MB Sapphire Radeon HD 4850
* Optical drive: DVD±RW Optiarc AD-5200S
* Card-reader: Sony MRW620
* System case: IN-WIN IW-S627TAC
Gaming Computer 2:
* Processor: Intel Core i7-920 (2.66GHz)
* Mainboard: Gigabyte GA-EX58-UD3R (iX58 chipset)
* System memory: 3x1GB Samsung (PC3-10666, 1333MHz, CL9)
* Hard disk drive: 1000GB Seagate Barracuda 7200.11 ST31000333AS
* Graphics card: PCI-E 896MB Leadtek WinFast GTX 260 Extreme+ W02G0686
* Optical drive: DVD±RW Optiarc AD-7201S
* System case: IN-WIN IW-J614TA F430 (550W)
Wattage:
Whilst it is for the most part considered one of the most important factors, it is in fact not. A PSU may be rated at 750W, but if it's supplying half that power across the 3.3V and 5V rails, it's not much use. You should be looking for how much power the PSU is providing across the 12V rail(s)
Wattage = Voltage x Amperage. For example, if there is 30A on the 12V rail: 30 x 12 = 360W. The PSU is supplying 360W across the 12V rail(s).
Be careful when you see a high wattage PSU at a strangely low price. There is a reason the price is so low. Poor quality components? Ridiculously good testing conditions? (PC Power & Cooling for example test all their PSUs at 50c before rating them) Who knows how long it could last before it "blows up"? Is there over voltage protection so it doesn't take your whole system with it? I hope I got my message across, quality over quantity. Besides, we already know that wattage is not the most important factor, so don't get reeled in by that 1000W at $50.
12V rail(s) and wattage distribution:
More is better? Less is better? The truth is... it doesn't really matter. Whilst some people claim that since each rail of a multi rail PSU can provide less power and therefore there is a possibility of one becoming overloaded, whereas this would never happen on a single rail PSU. Now, not every component is going to be pulling power from the same rail, is it? So it is almost impossible to overload one of the rails. You are not the guy who is overclocking a Phenom TWKR to 7GHz under liquid nitrogen and running a couple of power hungry graphics cards at the same time. There, one of the rails may become overloaded. But for 99.9% of users, it's never going to happen, so don't let it influence your purchase too much.
Just look out for one thing, many brands rate the rails of their PSUs at the maximum current one rail carry. For example, this Xigmatek has four rails each rated at 18A, it's a 750W PSU. (18 x 4) x 12 = 864W. Obviously, that's impossible, not all rails can carry 18A at the same time. So try to find out from the manufacturer exactly what amperage the PSU can output along the 12V rails if your see ten 20A rails on a 400W PSU (Exaggeration)
Efficiency:
Efficiency is not going to affect the performance of the PSU. The power output of the PSU is lower than the power it is drawing from the wall. The higher the efficiency rating, the less power is wasted and less heat is produced. Although, it could save you some money in the long run. And it could give you an idea of the quality of PSUs.
Brand:
Although there are countless brands of PSUs out there, very few of them actually manufacture their own. Big brands like Corsair, Antec, Hiper, Thermaltake... are simply rebranders. So think about that next time you pay more for a Corsair VX than a Xigmatek who are both manufactured by CWT.
Here is a link to the different OEMs for different brands.
How much power do I need?
The wattage recommended nowadays for some systems is ridiculous. Your water cooled, overclocked GTX 295 SLi system may actually be using a lot less power than you think. No, you don't need that 1.5KW $750 PSU.
Take at look at these graphs, they show the total system power under full load for a number of systems. A lot lower than you thought, huh?
Office Computer:
* CPU: Intel Pentium Dual-Core E2220 (2.4GHz)
* CPU cooler: GlacialTech Igloo 5063 Silent (E) PP
* Fan: GlacialTech SilentBlade II GT9225-HDLA1
* Mainboard: Gigabyte GA-73PVM-S2 (nForce 7100 chipset)
* System memory: 1GB Samsung (PC6400, 800MHz, CL6)
* Hard disk drive: 160GB Hitachi Deskstar 7K1000.B HDT721016SLA380
* Optical drive: DVD±RW Optiarc AD-7201S
* Card-reader: Sony MRW620
* System case: IN-WIN EMR-018 (350W)
Home PC:
* Processor: AMD Athlon 64 X2 5000+ (2.60GHz)
* CPU cooler: TITAN DC-K8M925B/R
* GlacialTech SilentBlade II GT9225-HDLA1
* ASUS M3A78 (AMD 770 chipset)
* System memory: 2x1GB Samsung (PC6400, 800MHz, CL6)
* Hard disk drive: 250GB Seagate Barracuda 7200.10 ST3250410AS
* Graphics card: 512MB Sapphire Radeon HD 4650
* Optical drive: DVD±RW Optiarc AD-7201S
* System case: IN-WIN EAR-003 (400W)
File Server:
* Processor: AMD Athlon 64 X2 5000+ (2.60GHz)
* CPU cooler: TITAN DC-K8M925B/R
* GlacialTech SilentBlade II GT9225-HDLA1
* ASUS M3A78 (AMD 770 chipset)
* System memory: 2x1GB Samsung (PC6400, 800MHz, CL6)
* Hard disk drive: 250GB Seagate Barracuda 7200.10 ST3250410AS
* Graphics card: 512MB Sapphire Radeon HD 4650
* Optical drive: DVD±RW Optiarc AD-7201S
* System case: IN-WIN EAR-003 (400W)
* Hard driives: 3 x 74GB Western Digital Raptor WD740GD
Gaming Computer:
* Processor: Intel Core 2 Duo E8600 (3.33GHz)
* CPU cooler: GlacialTech Igloo 5063 PWM (E) PP
* Mainboard: ASUS P5Q (iP45 chipset)
* System memory: 2x2GB DDR2 SDRAM Kingston ValueRAM (PC6400, 800MHz, CL6)
* Hard disk drive: 500GB Seagate Barracuda 7200.12
* Graphics card: PCI-E 512MB Sapphire Radeon HD 4850
* Optical drive: DVD±RW Optiarc AD-5200S
* Card-reader: Sony MRW620
* System case: IN-WIN IW-S627TAC
Gaming Computer 2:
* Processor: Intel Core i7-920 (2.66GHz)
* Mainboard: Gigabyte GA-EX58-UD3R (iX58 chipset)
* System memory: 3x1GB Samsung (PC3-10666, 1333MHz, CL9)
* Hard disk drive: 1000GB Seagate Barracuda 7200.11 ST31000333AS
* Graphics card: PCI-E 896MB Leadtek WinFast GTX 260 Extreme+ W02G0686
* Optical drive: DVD±RW Optiarc AD-7201S
* System case: IN-WIN IW-J614TA F430 (550W)
