How much UPS power do you need in case of an outage?
This question is important because the customer needs to decide which computers they want to protect and how long an outage they need to be able to ride out.
As an example, let's say the customer needs to keep core servers going -- and a handful of key workers.
You can protect each individual computer for a short while -- say, up to half an hour, depending on the model -- with a UPS. Servers will need to run until the last client is shut down, and so will need heavier-duty UPSes. But if your customer needs computers to be able to ride out long outages, you will need to look beyond UPSes and assess the viability of providing power via generators.
Bear in mind that if the electric supply is flaky enough to cause business problems, it is usually best to keep vital Internet sites on professional Web hosting services rather than running them in-house.