You need enough watts to supply the equipment you want to keep running. It is usually referred to VA (volt-amps), which is equal to watts in a simple case. Computer power supplies are not "simple", so VA is more accurate. If you get a UPS that can supply more VA than you need, that usually means you get more run-time. You will have to decide how much run-time you want. If you plan on just doing safe computer shut downs as soon as the power fails, all you need is 5 minutes or so. But if you want to try and keep things running until power returns (in a reasonable time), then you need bigger batteries. APC is a very popular brand, with MANY models & sizes.