I got 2 raspis for Christmas by accident. I'm using one to make my super-cheap accelerometer for earthquakes, and I decided to use the other for monitoring my APC UPS, I got from Costco.
These things are tricky. First, you must reach to the back of the pile to get a fresh one. Lead-acid batteries must be refreshed in storage, and they don't do that for these. Then you must check that the voltage is over 12V before you plug it in. This can be done with apcupsd on your main Linux computer. I got a bad one that had been left too long the first time. For some reason apcupsd works perfectly on my main Debian machine, but the raspi has proven to be a pain.
You really need this in the modern age. I have my modem, router, and ooma phone on it. It will last 3 hours with this, which is enough time to communicate that you are fine after an earthquake. If I put a big computer on it, it hardly lasts at all, so the raspi is perfect. Don't forget that the old phone line always worked in a power outage, but with an earthquake, it always overloads.
The key to making it work is to follow this link, for multiple UPS's even though you only have one. The automatic feature does not work for the raspi. Once you get it working, you can configure it to send out emails, etc. I'm just using it for logging.