FWIW, I have my Ademco Vista 20P alarm panel integrated with SmartThings via an Arduino and AD2Pi from Alarm Decoder. The alarm panel detects when its on its battery and I have mine setup where it will alert me when it goes from AC to battery and battery back to AC. I get this notification immediately plus the added benefit of having my hardwired alarm panel integrated into SmartThings. Full disclosure I have a UPS that my cable modem, router, and SmartThings hub are plugged into which keeps it alive during short power outages. The ADT alarm panel integrated is is documented here:
Stan and I are close to publishing a new version that will include the feature above.