They claim to raise 1 gallon by 1 ppm each second under ideal lighting. That equates to roughly 10,000 gallons raised 2 ppm over the course of a sunny day, or a little under 3 oz of chlorine a day. That makes their unit quite small compared to most of the more common units, even if you assume ideal lighting. The solar cells will also lose efficiency over time, which will cut into your chlorine production noticeably after several years. That means replacing the entire unit, not just the cell, which makes the price rather unattractive in the long run.