The only real variances in efficiency you are going to find are whether there is a method incorporated into the design to keep your UV glass tube clean or not. Otherwise you have efficiency control through circulation rates. There are minimum contact times required based on whether you are trying to control algae, bacteria or parasites in a motive stage. There are models that have quartz glass sleeves over the UV tube, I would not consider model with out one. There are models with wipers to keep the tube clean. Your best bet is usually with a wiper model and using a gate valve to control flow through the unit. Models with twisting passages and such really do not perform proportionally better in relationship to their increased costs and chiefly they just increase contact time which is best controlled with a gate valve and/or a properly sized feed pump. More expensive models charge a lot for supposed easier bulb replacement, but those benefits usually are not perportional to the increased prices. I would consider a 40 watt before a 25 watt. Most people flow water to fast and do not keep unit clean enough to perform near as well as a unit is advertized. Remember a lot more UV tubes are broken by taking units apart to clean them than wear out due to age. Wiper models cost about half again more than non wiper models, but you can consider a 25 Watt model with a wiper model. Figure on flowing water at one third to one fourth of rated flow for ideal performance if for bacterial and parasite protection. UV sterilizers are expensive and grossly over priced, but they definately do there jobs very well. Do it your self models are not difficult to make, but usually difficult to keep clean as most are made with glass use a lot of convolutions due to the typical use of multiple tubes (which are cheaper and more readily available), and usually not very safe as one eyes should be protected from UV lights.
