Indeed, passing data from the secondary to the primary of the transformers is actually hard, adding the handling of voltage transient spikes would make it even

Indeed, passing data from the secondary to the primary of the transformers is actually hard, adding the handling of voltage transient spikes would make it even more expensive. And you need only a handful of bytes per hour (assuming nice reporting per each hour) - real data is likely fewer than 16bytes (id-time-value), adding an iv and block size encryption is all it takes.