12V stability and ripple
Of the three voltages that power supplies offer, the 12V-rail is the most important one: 3.3V and 5V are less commonly used. Aside from the maximum deviation it is desirable that the voltage drops as little as possible at higher load, because that can result in an unstable power supply (and with that an unstable system). For regular users this is less relevant, as long as it is not too severe. However, for an overclocker it is necessary to keep the change to a minimum. The power supply with the smallest drop is the Seasonic Prime 650W Titanium, followed by the Silverstone Strider Titanium 600W and the Corsair Cs650M.
Also of importance is the ripple, which indicates to what extent the 12 volt rail fluctuates. This value is measured in mVtt and shows the difference between the highest and lowest measured voltage. A ripple of 100 mVtt means that the 12 volt rail fluctuates between 11.95 and 12.05 volt, if we assume a measured value of 12V. According to the rule, a ripple of under 75 mVtt is seen as acceptable, whereas a value of under 50 mVtt is good. Ripple is important for overclocking as well. The lowest ripple-value is accomplished by the Corsair RM650x, that reaches a maximum load of 7 mVtt. It’s followed by two Seasonic power supplies, the Prime 650W Titanium and the M12II Evo, with a maximum ripple of respectively 13 and 15 mVtt.