GPM stands for gallons per minute and refers to the amount of water tankless heaters can process per minute. When it comes to heating water, a tankless unit capable of processing 4 gallons of water (4 GPM) will comfortably run a couple of quick showers, suitable for a household with two or three occupants but won't be able to heat water constantly. For a continuous supply of hot water, a tankless unit with 6GPM, such as the EcoSmart ECO 27, is recommended. The ECO 27 is ideal for families with up to 5 members living in a two-bathroom house. Another solution for endless hot water is that Rheem RTGH-95DVLN – a High Efficiency 9.5 GPM Indoor Natural Gas Tankless Water Heater. This tankless heater has a stainless steel condensing heat exchanger, a built-in condensate neutralizer, and 1/2" Gas line compatibility up to 24ft.