Here's my canned text on sizing and buying a PSU:
Use the eXtreme PSU Calculator Lite to determine your minimum power supply unit (PSU) requirements. Plug in all the hardware you think you might have in 2 or 3 years (extra drives, bigger or 2nd video card, more RAM, etc.). Be sure to read and heed the notes at the bottom of the page. I recommend setting Capacitor Aging to 30%, and if you participate in distributive computing projects (e.g. BOINC or Folding@Home) or extreme 3D animated gaming, I recommend setting both TDP and system load to 100%. These steps ensure the supply has adequate head room for stress free (and perhaps quieter) operation, and future hardware demands. Research your video card and pay particular attention to the power supply requirements for your card listed on your video card maker's website. If not listed, check a comparable card (same graphics engine and RAM) from a different maker. The key specifications, in order of importance are:
Don’t try to save a few dollars by getting a cheap supply!
- Current (amperage or amps) on the +12V rail,
- Total wattage.
Digital electronics, including CPUs, RAM, and today's advanced graphics cards, need clean, stable power. A good, well chosen supply will provide years of service and upgrade wiggle room. Look for power supply brands listed under the "Good" column of PC Mechanic's PSU Reference List
. Note that some case retailers “toss in” a generic or inadequate PSU just to make the case sale. Be prepared to “toss out” that supply for a good one with sufficient power.
Most PSUs have an efficiency rating of around 70%. This means for every 100 watts of power a PSU draws from the wall, only 70 watts is delivered to the motherboard, with the rest wasted in the form of heat. The best supplies are 85 to 90% efficient, and as expected, cost more. I strongly recommend you pick a quality supply with an efficiency rating equal to or greater than 80%. Look for 80 Plus - EnergyStar Compliant
Too big of a PSU hurts nothing but your budget. Your computer will draw from the PSU only what it needs, not what the PSU is capable of delivering. If a computer needs 300 watts it will draw 300 watts regardless if the PSU is a 350W, 650W, or 1000W PSU. In turn, the PSU, regardless its size will draw from the wall only what it needs to support the computer. In this example, it will draw 300 watts, plus another 45 – 90 watts, depending on the PSU’s inefficiency.
As noted, the eXtreme Calculator determines the minimum
requirements. If the calculator (with the changes I suggested) recommends a 400 watt minimum, a quality 400W supply will serve you just fine. But a quality 550W – 600W supply will have, among other things, larger heat sinks to dissipate potentially more heat. It might have a larger fan too. The 400W supply will run most of the time closer to capacity, while the larger supply will be loafing along, rarely breaking a sweat. To help the smaller heat sinks get rid of the wasted 80 watts (20% of 400) of heat, the fan in the 400W supply may need to run full speed, while the fan in the larger supply, with bigger sinks just loafs along too – but in near silence.