Power Supply Capacity Formula

Solution 1:

Add up the wattage on all the labels and round up. According to PC Power & Cooling:

• AGP Video Card 30W - 50W
• PCI Express Video 100W - 250W
• Average PCI Card 5W - 10W
• DVD/CD 20W - 30W
• Hard Drive 15W - 30W
• Case/CPU Fans 3W (ea.)
• Motherboard (w/o CPU or RAM) 50W - 150W
• RAM 15W per 1GB
• Processor 80W - 140W

They also state:

For overall power supply wattage, add the requirement for each device in your system, then multiply by 1.5. (The multiplier takes into account that today's systems draw disproportionally on the +12V output. Furthermore, power supplies are more efficient and reliable when loaded to 30% - 70% of maximum capacity.)

Solution 2:

eXtreme Power Supply Calculator Lite

http://extreme.outervision.com/psucalculator.jsp