As a rule of thumb, the UPS should at minimum meets the watts of the devices connected to the UPS. For example, if a computer with a 1000 watt power supply is connected to the UPS, the UPS should be 1000 watts minimum. The greater the UPS, the UPS will be able to provide power for a longer period of time.
It is important to recognize that a UPS will only supply.power for minutes (perhaps 5 - 10) minutes. A UPS is meant to prevent equipment from losing power due to an unexpected loss of power. A UPS is meant to buy you just enough time to either safely power down the equipment or to engage an alternate power source, such as a generator.
A UPS can be expensive, so you may want to determine how many watts your equipment is actually using before purchasing a UPS. A multimeter can be used to determine the actual watts being used.