I'm the original poster.... In reply to your question....
First, I was mistaken on a couple of the details...
We have nine sites. And adding two more by April of 2022.
Each site has internet either provided by the site owner, or we brought it in via an Ubiquity 5 gig microwave shot. The Ubiquity, the Cisco router, the laptop power brick, and a few other items are run off a 600w APC UPS that has no internal battery but is connected to the 12 volt site battery (400 amp-hour) which has it's own Samlex charger.
We plan on running a Pi off of a buck converter off of the site battery. The Pi will be connected to the router via an ethernet cable (preferred) or wireless - each site has an old residential Dlink, Linksys or some other brand only for wifi-cellphone use while we are on site. (when you can pick them up for $5 each at thrift stores, why not?)
We will use a wall wart feeding a DC relay as the local AC power sensor... dry contacts grounding a GPIO pin. When the Pi sees the power go away it starts
a routine that checks every minute for 5-10 minutes to see if it's a momentary or a "hard" outage (could be as short as 10 min, could be a week... a few years ago one forest fire burned out the power lines to one site and the site was on generator for a month. Someone had do a round trip of 30 miles of highway plus 15 miles of 4x4 roads every other day for 51 days with ten 5-gallon jerry cans of gasoline on each trip.
On a power fail the Pi will to send an email message to a list of addresses (probably 8 to 10 addresses)...
The email will contain the site name, the local time, date, day of week, system uptime, and any other Pi-connected sensor info... Inside temp, outside temp, humidity, etc.
On a power return, it depends on if the duration exhausted the battery (which is primarily there for the repeaters). Each site has a low voltage disconnect circuit to prevent killing the batteries with overdischarge...
If the Pi is still running (i.e. the disconnect didn't activate) then the system will send a "power is restored" email similar to the above.
If the disconnect did activate and the Pi has to reboot then it will note the parameters for the restoral email (time, day and date, etc from it's RTC board), then wait until the microwave internet link re-establishes (i.e. wait until it actually can send the email).
Once the link is up it pings the us.ntp.org server to verify the link is actually up (and BTW to verify the actual time and date). Then it sends the email, with a note as to the time of the actual power return.
And a cron job will send the periodic status message that I mentioned in the original posting.
Again, I don't mind buying something, even ham built, as long as there isn't a monthly charge and it's a stand-alone device (i.e. not cloud based).
One person pointed me to this: https://www.amazon.com/gp/product/B089QQNKJL
It's financially attractive, and the fact that is has three AAA cells inside to make 4.5 volts to power it while the power is off can be worked around... The three cells could die and leak... we've not been to some of the sites in years.
The workaround is two pieces of wooden dowel the diameter of a AAA cell, with a brass thumbtack pressed into one end and a wire soldered to the thumbtack. You can get 4.5 volts from a buck regulator off the site battery.
The biggest issue with that device is that there is a company monitoring it and actually sending the emails, and cloud companies have a habit of evaporating.
Since I posted the original message we've been focused on other things... we had to move 3 systems from one building to another at one mountaintop, we had to replace 4 antennas at another mountaintop, and the Thanksgiving holiday happened (my wife decided that we were going to do a Thanksgiving trip to another state... I drove 600+ miles / about 12 hours each way and we spent a week visiting family).
I'm probably going to be building up something using a Pi ver 2, a metal case (https://www.amazon.com/Flirc-Raspberry-Pi-Zero-Case/dp/B08837L144),
and using the ideas presented in the previous postings.
Again - comments, suggestions, pointers to products - even a 1-man ham company in a garage - are welcome.
Mike WA6ILQ (callsign //at// gmail //dot// com)