**broken link removed** looks pretty good,
especially this feature: "Embedded TCP/UDP stack - allows you to upload data to a web server.".
I'm assuming the Brown/Blackouts can/will occur in multiple locations and that multiple sensors may trigger
simultaneously, so the Web site you create would, of course, need to be able to handle potential pile-ups.
And between the Uno and the shield there are ample GPIOs and ADCs to accommodate additional data beyond the basic ON/OFF conditions you wish to monitor.
Also, the device noted above can pull significant current (2A). But from the specs, it appears that for simple SMS traffic the draw is considerably less. Nonetheless, it should have its own Vcc connection to the battery, i.e., NOT derive its power from the UNO pass through power pin.
This
YouTube video was useful.