How much water can you purify on a set of CR123 batteries in a Steripen? My MIOX pen is rated for 200L on 2 CR123s. I've never actually measured to see if that's realistic or not but if it is, I can carry 6 spares in a small case and have a backup to boiling that can handle over 750 Liters of water! That's well more than a couple of months worth.