bitlocker is not trusthworthy IMHO; I'd check out
VeraCrypt - Home
you might be able to store the encryption key on a usb drive hidden in a wall on a long usb extension.. then if they yank all the cables and take it the'll leave the key behind and it'll still recover automatically..
Power outages can last longer than your batteries; and with Murphy's law in full effect combined with Ocams Razor it seems more likely of an event to occur than data being utilized; I suspect if someone walks out with your NVR the'll just destroy the disks if they are this technically savvy.. as they want to destroy evidence; not retrieve it and put it on the internet to incriminate them selves.
If you go the full disk encryption route I would have a backup solution in place; so if/when your NVR is sitting there asking for a password you can still have something recording somewhere.
I admire your drive (pun intended) to encrypt all this stuff; but you likely have bigger fish to fry than plug this minor attack surface.. you think a few hour long power outage wont be a problem but potentially your own video surveillance recordings being physically stolen and used against you is likely? Not sure I want to ask what your doing to be this paranoid.. but I can see the desire to keep the courts from using em against you if something crazy happened one day; in that case you can only store the decryption keys in your head.. as thats the only place they are safe from legal reach (5th Ammendment)