First, I tend to view UPS like anti-virus, they are “best practices” or simply good computer hygiene, that help to prevent costly failures.

Second, It depends on your situation. There are those who will disagree, but I believe that in most of what we call the “first world”, the power grid is reliable enough that a UPS doesn’t provide much additional protection beyond what a good surge protector will give you. A rare power failure isn’t going to damage most things.  But a UPS does more than just protect your hardware, in fact it is there to also protect your data, and software.

So it really comes down to you: what are your work habits, and how much is your time worth?

If you figure a good, cheap UPS is around $100 then, over the life of that (say, 3 years), given the power reliability of where ever you live, what’s at risk? Do you work for hours before saving, or save every 2 minutes? Does your power go out rarely, or a few times a month? How much do you worry about losing components to a power surge or brownouts?

How much is your peace of mind worth?

Where a UPS really shines is in situations where you have a lots of small brownouts — the power doesn’t cut out entirely, but it wasn’t all there for a brief moment either. This can happen in places where the power is less reliable. For example, you may be in an older building, a very rural location, or in a country where the power situation is less stable. In that case, a UPS will definitely pay for itself quickly, and you want to get one that not only provides stand-by protection but also what they call “sine wave output”.

And, of course, you might also be going for a high-availability solution. In that case, a large UPS (and maybe even generators) is a must.

Computer power supplies are switching power supplies that can handle small (~20V) variations in power automatically. Newer computer power supplies that are high efficiency (Active PFC) may even have a wider operating range.

That said, when the line power is outside (above or below) the standard range, the power supply may fail to deliver the right voltages (Volts) at the necessary or expected current (Amps or Amperes) levels. If the power supply fails such that it delivers either too high a voltage or current, this may damage the components in the computer, including low voltage, “high” current devices such as the CPU and RAM.

Two devices that are related but do not replace an UPS are:

• Surge Protector
• Line or Power Conditioner

These devices are generally simpler and thus less expensive than an UPS.

That said, I personally and professionally always try to use UPSes to protect the computers I use. The cost of a good UPS is about 10-20% of a mid-range desktop system, but a decent UPS with replacement batteries can last 10+ years of services so the cost it amortized over quite a numbers of years, and in my own personal (and professional) experiences every single UPS I have bought has paid for themselves either in cost savings in terms of preventing premature hardware failure, or in terms of cost of preventing data loss.

The batteries do need to be replaced 3-5 years depending on circumstances (frequency of battery usage, battery chemistry, avg/peak temperature, discharge amount), but are $40-50 per UPS for an average mid-range system suitable for a home or office desktop system.