When we started engineering we were planning to use rechargeable batteries, much like you get in an electric toothbrush. However, because you only need to use the Loogun for short bursts in order to clean the toilet, the actual power usage is very low per use and so we chose AA's instead.
During our interval testing we found that fresh high-drain batteries, running for an average time of 30 seconds per use, last for around 180 uses, or around 6 months if used once a day.
A list of recommended batteries is provided on our blog. It's important to always check that the batteries you're using are specifically designed for high-drain devices.