Power source event trigger

Late 2014 Mac mini running Sierra connected to an APC BackUPS Pro 900. No special software has been installed to support the UPS. At the moment I run pmset every minute to determine the power source, and trigger actions based on the results. However, a dialog window pops up as soon as the Mac switches to backup power. I'm wondering if there is any way I can trigger based on anything else, given that macOS seems to be able to do so.