Hi,
My backup is a separate chart plotter with it’s own dedicated radar and sounder. I run Navionics charts on the main system and C Map charts on the back up system. A lightning strike will probably take out both systems at the same time, leaving me with my Navionics apps on the phones and tablet. If we sail near lightning, we put the phones and tablet in the microwave oven, Apparently the steel casing in the microwave protects the electronics from lightning. (I’v yet to test this theory). But I know of a few vessels in Darwin that have sailed home on a iPhone Navionics app after lightning has wiped out the electrics and electronics on board.
I have OpenCPN on my laptop, but tend not to use it, because I’v had no issues using iPad Navionics outside mobile coverage. OpenCPN works fine and its free.
Cheers, Ross