If the battery inverter in the Anker box doesn’t pass through grid power then I think you would use an automatic transfer switch that switches between mains and battery inverter depending on which is powered. I had dreams of offsetting my homelab power with solar + battery + inverter.
I’m pretty sure this will be a lot of work. Even assuming you didn’t rename any files and all the torrents are in a single folder, you still need to re-search for every torrent file and add them to a torrent client that should automatically find the files in the target location and check them to see that it’s already downloaded. I’m not sure if all torrent clients do this so I would advise testing it first - when re-adding a torrent with existing files I usually add it paused and then force re-check to be safe.
In theory it should be possible to make a program that does this - automatically indexes and crawls torrent sites and DHT, fetches torrent meta info, then scans your HD for matching files to seed. This feature is a proposed one for the DHT crawler “bit magnet” so I’m hopeful that one day there will be a program that can do that.