Battery capacity is related to discharge rates. I think Panasonic publishes data.

I bring two sets of batteries and two lamps on overnight rides. On little local training rides where I know the roads, I usually just have one set on the bike.

If on unfamiliar roads or when going briskly on said overnight rides, I keep one of the lights on low power (100 lumens) and the other I switch between various power levels (200 lumens or 400). You have not lived until you had lights go out on a moonless night on mountainous descent. 100% redundancy is my fix. My two setups weigh less than most generator setups and I do not consider it overkill. I know I will have light because I have two of everything.