Power-- why bother about it?
Electrical (power) company, at least in my area, uses a 110 VAC ideal and +- 10% as normal. So, 100 VAC to 121 VAC range is normal on a circuit coming into house. This does not sound too bad.
Now, if a house is air conditioned the AC unit can draw such a surge of draw at startup that the whole house experiences a sag (voltage drop) of 5-10% MORE and then cause a surge of that much. A refrigerator can cause something similar if compressor is old and/or coolant pressure is low due to a leak.
So, power is for whatever reason under the ideal voltage, and AC cuts in. Power variance can be such that the voltage coming into a computer power supply is 80-90 VAC for a short time, then suddenly 120-140 VAC, then back to about 100 VAC. A PSU (literally means Power Supply Unit) is not a power conditioner with a battery and lots of MOVs to allow for something big to draw power of even 110% from repeatedly and for uncertain durations.
It has caps (capacitors, "cans" that act as very short term batteries that do drain themselves if not power to them or less power than what is being pulled out of them) that if computer is on will drain completely in parts of seconds at max if power stays down. If power surges beyond limits it gets damaged (even by surges that are slightly over limit if it gets many over time).
Now, lets get to the "nitty-gritty"-- components in a computer are (by specification) designed to vary 5% to 7%. Nice, reasonably accurate circuit, UNTIL the power coming into PSU varies. Then, given the above, the voltage coming into a circuit can be out of specification by 3 to 4 times the acceptable variance as it enters computer.
Surges, Handling of:
The best surge suppressors are not cheap. They have to dump surges to ground to protect computer power supply. They have to do so in such a way that no draw from one outlet causes a sag of large amounts in another outlet (so they have to be used within the rating they are designed to supply). They have to react very fast. They have to die if need be, and in such a way that they do not catch fire. The surge suppressor you might stick on your air compressor will not do. Oh!! Ideally, they should keep power from going more than 10 percent higher than normal for more than about .2 seconds at any time. Cheap ones clamp at 330 VAC, which is 300% or three times normal.
Sags, Handling of:
Sags are when power is "not up to par" voltage wise. If a circuit designed for 105-110 VAC input and 12 VDC out is used on a circuit that often offers 80-90 VAC to it, unless it has an extra source of power that it is designed to use (think battery), it puts out 10.5-11.5 VDC if well designed for the time the sag lasts, and the longer the sag lasts the more the caps (think very low capacity batteries) are likely to not be able to provide what is needed to cope and keep the voltage at the voltage it needs to be for the computer to work right(within 7% +- for consumer stuff and within 5% +- for milspec stuff). We are talking parts of seconds here, not minutes or hours for a modern computer (used to be seconds).
The best solution is a battery, or batteries-- in a device called a power conditioner or UPS. UPS has bigger batteries, bottom line, and can usually be adjusted as to how sensitive it is and probably alarm with loud noise if battery is being drained or close to empty, and the best ones come with software that will shut down computer if battery is critically low and something like a generator has not cut in and started feeding power.
Interference-- Usually an electrical pattern that is not wanted which "rides" the same conductor as a power input flow of wattage at a certain voltage. Interference causes electronics to misunderstand things and to mis-react if it is too great.
Power Conditioner-- A device that is in-between a surge suppressor and a UPS. Typically, Power Conditoners use very small batteries or very large capacitors or both to fill in (or bridge the lack of correct voltage) and bring an undervoltage up to spec for a very short time.
Power Voltage Variances-- power can be at ideal voltage, undervoltaged (which some people speak of as continuous sag or brownout or simply say it "is sagged"), or overvoltaged (also spoken of as "surging", power "spikes," or in a "surged" state).
Sag-- undervoltage that is out of acceptable range.
Surge-- overvoltage that is out of acceptable range.
UPS-- Uninterruptible Power Supply, which is a misnomer because the battery capacity determines how well a UPS can work, and how fast a UPS can switch to battery is also a factor.
John C. Danielson, II