All about output
Manufacturers usually list their power supplies' output in watts. A higher-watt PSU can supply more power. Desktop power supplies have a power output rating of from 200 watts to 1800 watts (for ultra-high-end, enthusiast-class products). Wattage ratings higher than that would exceed the capabilities of a typical 15-ampere electrical outlet. The important number here is the one for sustained or continuous power, not the one for peak power. Most power supplies can operate at peak power for only brief periods.
Ideally your unit will delivers plenty of power to your components and offers some extra headroom in case you want to attach additional components later. Most power supplies hit their peak efficiency levels with loads in the range of 40 to 80 percent. Building to about 50 to 60 percent of a PSU's capacity is advisable to achieve maximum efficiency and yet leave room for future expansion.
For example, if the maximum power or combined TDP (total design power) of your system's present components is 300 watts, a 600-watt PSU would be a good fit. In a high-end system loaded with components that may peak collectively at 700 watts, a 1200-watt PSU would work well. You can get by with lower-capacity units if you don't think you'll ever need to expand your system, but if you can afford it, choosing a higher-capacity PSU is a better bet.
Outervision and Thermaltake's handy-dandy PSU wattage calculator invites you to input your build components in exacting detail--right down to CPU overclocking voltages and specific water-cooling components--and then spits out a ballpark power-supply wattage for your system.
On the subject of wattage, one common power-supply myth holds that higher-wattage power supplies necessarily consume more power. Untrue. All else being equal, a 500-watt power supply won't consume any less power than a 1000-watt unit. That's because a system's components--not its PSU--dictate its power consumption. If you have 300 watts' worth of components in a system, the system will consume 300 watts under load, regardless of whether the system is outfitted with a 500-watt power supply or a 1000-watt one. Again, a PSU's wattage rating indicates the maximum amount of power the unit can provide to your system's components, not how much power it consumes from the outlet.
An efficient PSU is a better PSU
A power supply's efficiency rating is important because higher-efficiency units tend to have better components, waste less power, and generate less heat--all of which contribute to less fan noise. A power supply with an efficiency rating of 80 percent provides 80 percent of its rated wattage as power to your system, while losing the other 20 percent as heat.
Look for units with "80 Plus" certification. Though the certification process isn't especially stringent, 80 Plus-certified units are confirmed to be at least 80 percent efficient; and 80 Plus has tiers for even more-efficient units, including 80 Plus Bronze, Silver, Gold, Platinum, and Titanium certifications. Power supplies in the higher certification tiers tend to command very high prices, however. Average users with average needs should probably stick to the simple 80 Plus or the 80 Plus Bronze level unless they find a particularly juicy deal on a Silver or Gold PSU.