SteriPEN with 4 AA batteries.
This sterilizes clear water, we never had serious trouble with it. However it's on-off switch was becoming very questionable so we took it back to MEC. We replaced it with the lighter (130g with battery) Adventurer model. This solved the two short-comings:
1) It is very difficult to see it is on in the desert sun (new model flashes a LED light when it is sterilizing)
2) The electrical water sensing device that protects you from turning the device on when not in water has been replaced by a more reliable optical water sensing device.