There are quite a number of things that can go wrong with your electrical service that can adversely affect your computer. The first and most obvious is a power failure, where all electrical power is lost.
Power failures fall into two categories. The distinction is whether they last less than 6 seconds, or greater. Power failures of less than six seconds are when a circuit breaker has tripped in a substation somewhere, and the circuit breaker was able to reset itself. Power failures of greater than six seconds are when the circuit breaker cannot reset itself, and an operator must intervene. This can involve downed power lines, fires, etc., and can take quite some time to resolve. Obviously, a computer cannot operate with no electricity, but a power failure, in and of itself, will not harm your computer. Computers can, however, be damaged by the condition of the power immediately before and immediately after the power failure. There is also the potential for lost data and corrupt files.
Another power hazard is variations in the voltage level: overvoltages or undervoltages. These typically occur when other devices on the same circuit (not necessarily inside your house or apartment) draw either significantly more current than normal or significantly less. For example, construction in your neighborhood or a stuck elevator in your building can cause either of these conditions. Undervoltages can also be caused by your utility company, and are called "brown-outs". These can cause components in the computer to run too hot, or affect the timing between devices.
A large, quick overvoltage is another power problem, and is called a surge. Surges are caused by lightning strikes and failures in electrical distribution equipment, and frequently occur before and after power failures. Surges can overload components and cause them to fail.
Line noise is when a high frequency signal is riding on the frequency of the alternating current. With Direct Current, or DC, the electrons are constantly traveling in the same direction through the circuit. With Alternating Current, or AC, the electrons are constantly changing direction, 60 times a second or 60 Hertz (Hz), within the circuit. Other devices on the circuit, such as radios, electric motors, and even other computers and monitors, can generate higher frequency variations within the 60Hz AC. If the frequency of the line noise coincides with the frequency of any of the components of the computer, it might affect the operation of those components.
Surges and Line Noise
Surge suppressors are designed to divert surges away from your computer equipment and filter out line noise. In general, they do a good job, but quality varies greatly. Some surge protectors will divert the first few surges, but let subsequent surges through without notifying you of the condition. The best indicator of quality is reputation. T C Solutions recommends Tripp Lite products, though there are a number of quality products on the market. A $6.99 surge suppressor at your local home improvement superstore probably isn't a good way to protect your $2000 computer system. Expect to spend $40-$70.
Overvoltages and Undervoltages
Voltage regulators can protect your equipment from these hazards, but so can some UPSs. Consider a Line-Interactive UPS.
To protect against power failures, you can employ an Uninterruptable Power Supply (UPS) which, in its simplest form is a battery backup. UPSs come in three flavors: Backup, Line-Interactive, and On-Line. All three include surge suppression and Line Noise Filtering.
The Backup (or Standby) UPS
The Backup UPS provides backup power when the utility power fails. Utility power usually is from 115 to 120 volts. When it drops below about 90 volts, the Backup UPS switches its outlets to run from its internal batteries. When the utility power returns to above 90 volts, the Backup UPS switches its outlets back to the utility power, and at the same time, recharges its internal batteries so they will be available for the next power failure. The switching time is typically less than 1/100 second, which is fast enough for most computer equipment.
The Line-Interactive UPS
The Line-Interactive UPS adds a voltage regulator to the Backup UPS. In addition to providing battery backup, the Line-Interactive UPS will correct undervoltages and overvoltages. For most applications, this is the best protection available.
The On-Line UPS
The on-line UPS bypasses any problem that might arise due to the switching time associated with the Backup UPS by constantly supplying power to its outlets from its internal batteries, and only using the utility power to recharge these batteries. This eliminates the switching time of the Backup UPS, and is generally only required by high-speed telecommunications equipment and the healthcare industry.
What's a VA?
Good question. A VA is a Volt-ampere. Does that help?
Didn't think so. UPSs are available in various sizes. You need to know how much stuff you're going to plug into a UPS to determine how big a UPS you need. Add up the current (in Watts) of all the items you will plug into a UPS, and multiply the result by 1.4. If you have a computer with a 200 Watt power supply, and your monitor draws 100 Watts in normal operation, that's a total of 300 Watts. Multiply 300 by 1.4, and the result, 420, is the smallest UPS, in VA, that you should use.
Remember: It doesn't make much sense to have your computer plugged into a UPS, and not the monitor! How can you select the command to shut down the computer if you can't see your mouse pointer?
A computer with a 200 Watt power supply will probably not draw 200 Watts during normal operation. It will probably only draw about 90 to 120 Watts after it starts up (unless you're playing a 3D game. These games can cause a computer to draw quite a lot of power.) If the devices plugged into the UPS ever draw more than the UPS is rated for, then the UPS can be damaged.
A UPS under full load (for example: a 420VA UPS with devices connected drawing 300 Watts) will provide between 5 and 10 minutes of protection. That's plenty of time to save everything and shut down. You should not consider continuing to work (or play) because if the power failure lasts longer than 6 seconds, then it will probably outlast the capacity of the UPS.