The very first thing you need to know is how much power does the UPS System need to handle?
There are guides in this help section that will assist you if you don't know how to find this out. Note that if you have a varying load you need to be able to ensure that the UPS is big enough to handle the maximum load, even if it may only be at this load for a short period.
Once you have the power, think about the UPS technology. You've got the option of basically 3 types of technology and each have pro's and cons.
VIX. This means a line interactive square wave system. (Line Interactive UPS Systems will boost a low voltage and buck a high voltage without resorting to battery power). These square wave systems are low cost and suitable only for loads that are computer related. Used for modems, routers and home PCs they offer affordable power protection. Silent in operation although higher powered models may have a fan activate when they are active (ie on battery or bucking/boosting). Higher grade computers may not be suitable for square wave systems and may require a sine wave model.
VIS. This is line interactive as above but has the advantage of being sine wave in nature. This means this type of UPS is suitable for a wider range of applications including systems with motors and lighting, and those computer power supplies with power factor correction that can be susceptible to square wave UPS'.
VFI. This technology provides the highest degrees of power protection as the UPS System is permanently 'on'. As a result the system can handle a much wider input voltage swing, has a fixed high quality pure sine wave power output and can be extended in runtime with the addition of additional battery packs. These systems are also referred to as Online Double Conversion UPS Systems. The drawback is efficiency (although some systems can incorporate an 'ECO' mode to basically become a line interactive UPS) and the fact that the required forced cooling makes them noisy in operation and hence only suitable for computer rooms or other naturally noisy environments. Not suitable for a quiet office.
This is the main function of a UPS System of course, to provide backup power when the utility fails. Some people are surprised to find that most UPS only actually give a few minutes of runtime at full load, with many expecting 30minutes or even an hours runtime as standard. However the amount of runtime you will get is a function of the amount of power that you are demanding and the battery capacity. Long run time, high powered applications require a lot of batteries!
You can see what UPS will meet your application requirements by using the Uninterruptible Power Supply Selection Tool, with all you need to know is the power requirement you need. If you know what UPS you want and want to know what runtime you will get use the UPS System Runtime Calculator instead.