Ripple is the variation (regular, irregular, or both) about a steady-state DC voltage during steady-state electric operation. In a linear power supply, it is usually related to the AC mains. In a switching power supply, it is harmonically related to both the input line and the switching frequency of power converter stage. The amplitude can be expressed in millivolts peak-to-peak or RMS value of the combined line frequency and power converter frequency components.
Harmonic is a sinusoidal wave having a frequency that is an integral multiple of a fundamental frequency. For example, a complex wave whose frequency is twice that of the fundamental frequency is called the second harmonic. Harmonics in a power system cause distortion of the normal sinusoidal voltage waveform.
漣波是衡量直流電壓穩定度的指標，漣波越小者代表電壓越穩定
諧波則會扭曲交流電壓的波型(標準的交流電為正弦波)
不穩定的電源會影響電腦的正常運作，UPS不僅在市電中斷時可立即供應電力，在市電正常時也可對品質不良的市電作穩壓、濾除雜訊及防雷擊等功能以提供穩定純淨的電源。所以使用UPS就像為電腦設備買保險一樣，有備無患。