System capacities vary from as little as 2 to 5 gallons per day to as much as any household would need. The cost also varies, but the need for treated water should be the controlling factor. If the contaminant is a hazard only if ingested, water used for cooking and drinking needs treatment. A small unit that supplies a separate faucet at each location for water consumption is sufficient. A large home unit or small commercial unit is necessary.
Most major manufacturers produce units in a wide range of sizes.
The price and complexity of management increase with size. Small units have a throwaway membrane and filter cartridges. Larger units often have cleanable filters. Because they are too expensive to discard when partially plugged, they usually are cleaned several times before replacement. The larger units usually last longer, however, and produce higher quality water at a lower cost per unit.