You might want to look at some of the 'Pi UPS' systems which have been developed. Once they indicate the power has failed you can start a graceful shut down. On

You might want to look at some of the 'Pi UPS' systems which have been developed. Once they indicate the power has failed you can start a graceful shut down. One example can be found here: https://pi.gate.ac.uk/pages/mopi.html https://pi.gate.ac.uk/pages/mopi.html