Link: http://extreme.outervision.com/psucalculatorlite.jsp

Area 1 (System Type) -

Box 1 - Leave this as it is

Box 2 - Leave this as it is

Box 3 - Select CPU brand 

Box 4 - Select CPU socket (Google your CPU model if you don't know what it is) 

Box 5 - Select CPU model  

Box 6 - Leave this as it is

Area 2 (Overclock my CPU!) - This area is for overclockers so unless you know what to do ignore this area 

Area 3 (RAM) - This area is to select the number of RAM sticks you're going to use (eg: If you have 2 sticks of DDR3 RAM select the option "2 sticks DDR3 SDRAM")

Area 4 (Video Cards) - 

Box 1 - Select GPU brand

Box 2 - Select GPU model

Box 3, 4, 5 - If you have more than 1 GPU select them

Box 6 - Leave this as it is but for multi GPU setups if NVIDIA choose SLI and if AMD choose Crossfire 

Box 7 - Not important

Area 5 (Hard Drives) -  This area is to select the number of HDDs you have. 

IDE (or ATA): implies a controller that's on the disk drive itself and works with a narrow bus interface → different versions with max. around 100MB/sec transfer speed. 

SCSI: implies a serial interface were controllers are linked directly to the disk drive → 3.0Gb/s max. = 3 x 125MB = 375MB/sec. Used in BIG systems → multiple devices (reason it's not here on GD) 

SATA: S(erial)ATA implies a point to point connection with at least four wires → transfer rate up to 6.0Gb/sec (750MB/sec) with higher RPM's. 

Most common these days: SATA is used the most these days and I guess almost 50/50% run @ 3.0 respectively 6.0Gb/sec

(Source:Mark1311811)

Area 6 (SSD Drives) - This area is to select the number of SSDs you're going to use. If you don't know which type of SSD you have visit the product's home site and check. 

Area 7 (Drives) - This area is to select the ROMs you're going to use.

Area 8 (PCI Cards) -  This area is to select the PCI cards you're going to use. 

Box 1 - If you're going to use more cards than supported by the motherboard

Area 9 (PCI Express Cards) - This area is to select the number of PCI express cards you're going to use excluding GPUs. 

Area 10 (External Devices) - This area is to select the external devices that will draw power from the system you're going to use. 

USB - Things like potable HDDs and controllers. Exclude thumb drives.

Firewire - I don't think you'll be using them

Cold cathodes - Amount of cathodes used for computer case lighting.

Area 11 (Cooling) - This area is to select the cooling equipment you're going to use. 

Area 12 (Power Supply Adjustments) -

Box 1 - Leave this as it is

Box 2 - I recommend selecting 40% if your going to use it 24/7 for about three years.

Now you can click calculate and see the amount wattage needed for your PSU.

So did you find this guide helpful? Anything I missed? Spot any errors (excluding spelling and grammar mistakes :P)? Please leave a comment below.

Thanks for reading! :)