NUT is the Network UPS Tools. To setup NUT on Debian, you first need to know what kind of UPS you have, and how it is/will be connected to your Debian machine.
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We will not yet analyse how to do monitoring over the network, but only on the machine physically connected to the UPS, which we suppose has been installed under Debian GNU/Linux (tested on Sarge).
First, install the package nut (there are several other NUT-related packages, but you only really need this one).