Editing Mains electricity (section)

== Power quality ==
{{Main|Power quality}}

The stability of the voltage and frequency supplied to customers varies among countries and regions. "Power quality" is a term describing the degree of deviation from the nominal supply voltage and frequency. Short-term surges and drop-outs affect sensitive electronic equipment such as computers and [[flat-panel display]]s. Longer-term power outages, [[brownout (electricity)|brownout]]s and [[power outage|blackout]]s and low reliability of supply generally increase costs to customers, who may have to invest in [[uninterruptible power supply]] or stand-by [[engine-generator|generator]] sets to provide power when the utility supply is unavailable or unusable. Erratic power supply may be a severe economic handicap to businesses and public services which rely on electrical machinery, illumination, climate control and computers. Even the best quality power system may have breakdowns or require servicing. As such, companies, governments and other organizations sometimes have backup generators at sensitive facilities, to ensure that power will be available even in the event of a power outage or black out.

Power quality can also be affected by distortions of the current or voltage waveform in the form of [[harmonic]]s of the fundamental (supply) frequency, or non-harmonic [[intermodulation|(inter)modulation distortion]] such as that caused by [[electromagnetic interference]]. In contrast, harmonic distortion is usually caused by conditions of the load or generator. In multi-phase power, phase shift distortions caused by imbalanced loads can occur.