You need pressure on the feed side to overcome the osmotic pressure differential between the dirty and clean sides. The dirtier the dirty side gets, the higher

You need pressure on the feed side to overcome the osmotic pressure differential between the dirty and clean sides. The dirtier the dirty side gets, the higher pressure you need to move water from the dirty side to the clean side. Eventually you have to flush the dirty side because the concentration of contaminants is too high and nothing will go through the filter even though there's still water that you could use if you could get it through the membrane. Under-cabinet RO systems are less efficient because they rely on the relatively low-pressure water that's running to the tap and are made to work without electricity.