Battery Equivalents and Replacements

What is the Difference Between a Surge, a Spike, a Sag, a Brownout, and a Blackout?

For electric and especially electronic devices it is very important that the power supply is "clean", without over-voltages and under-voltages, but also that the AC power features pure sine waveform with minimum Total Harmonic Distortions (THD).

Words like surge, spike, sad, brownout, and blackout are often used to describe events on the power line, but what do they actually mean?

Published: May 24, 2022.

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What Are Spikes, Surges, Sags, Brownouts, and Blackouts?

Spikes, surges, sags, brownouts, and blackouts are all undesirable events on the power lines that can cause issues with the devices being powered. These events are either over-voltage events (spikes, surges) or under-voltage events (sags, brownouts, blackouts) that last for some time ranging from one-millionth of a second (or even shorter) to seconds or even hours, days, etc.

All electric and electronic devices are designed to withstand smaller instabilities in power supply, for example, ±10% of a nominal voltage. However, anything beyond that range can cause the device to malfunction, reset itself, or it can even damage the device.

Spikes vs Surges

Both spikes and surges are over-voltage events.

Power spikes are transient, very short events that may last one-millionth of a second or even shorter. Although line voltage can increase significantly, the total energy of the spike is relatively low, practically causing no issues with electric and electronic devices.

Power surges are over-voltage events that often feature lower maximum voltage when compared with power spikes, but the power surges last much longer than the power spikes - they are often measured in thousands of a second.

Electric and especially electronic devices are very sensitive to power surges and in areas where such events are common, the use of surge protectors is highly recommended, with 2000 Joules models recommended for electric devices and 4000+ Joules models recommended for electronic devices.

Note: Maximum current (Amps) and energy (Joules) of the surge protector define its ability to power certain device(s) (Amps) and to absorb the power surge (Joules) - the more the better, but also larger the size of the surge protector and its price.

Sags vs Brownouts vs Blackouts

Sags, brownouts, and blackouts are under-voltage events.

Sags are very short periods of lower-than-normal line voltage, which is short enough NOT to cause electronic devices to reset (TVs, desktop computers, and similar).

In most situations, when sag happens, an Uninterruptible Power Supply (UPS) unit will "click" and continue to operate as if nothing happened.

Brownouts are periods of low voltage or no voltage at all that may last for up to a few seconds, causing electronic devices to reset (TVs, desktop computers, etc.) and UPS units to turn ON their protection.

Blackouts are periods of low voltage or more often no voltage at all that may last for hours, days, and even weeks.

In order to provide power during the blackouts, one needs solar panels in combination with a deep cycle battery and solar battery charge controller and a power inverter, a solar generator/power station, and/or power generator.


Long Story Short: Spikes, surges, sags, brownouts, and blackouts do happen, but one can also be prepared for such events.

However, when trying to protect your devices from such over- and under-voltage events, don't go cheap - a weak surge protector can provide a sense of security, but in reality, the devices that it protects can be damaged or destroyed as well.