Obviously bitcoin couldn't be used in normal fashion with no electricity(but
you could trade physical bitcoins, thumb drives that contain wallets,
paper wallets).
Surely thumb drives and paper wallets could be copied?
Eventually we may have battery-powered secure hardware wallets that can exchange transactions via NFC or Bluetooth. Without a network there'd be no way to confirm them, so you'd have to trust the buyer wasn't double-spending, but standard hardware wallets wouldn't generate double-spends.