This is a great question, and one we are VERY happy to explain. Manufacturers have decided to inflate their perceived performance by listing a model’s MAXIMUM gallons per minute. Some even put the design max into their model number. For instance, a very popular Noritz tankless water heater is the NRC 111 DV. This model is rated for a maximum output of 11.1 gallons per minute at 120 degrees. Here is the “sneaky” part. To get 11.1 GPM out of the heater, the incoming water temperature must be 90 degrees. Incoming water temperature here in the Puget Sound is going to vary from 45-55 degrees. We use the 45 degree mark because we don’t want to “Over Promise, and Under Produce”. We would rather have you find out you are getting slightly MORE hot water out of the system than the other way around. To really find out what a tankless system will do here, you take the incoming water temperature (45 degrees) and subtract that from the desired output temperature (120). This tells you the degree of rise (75). Then go to the performance chart. The Noritz Chart below is figured for 120 degree water in mind, and shows that with these parameters, the NRC 111 DV will produce 5 GPM. You can also see that the NR 98 heater will give you 4.5 GPM with the same numbers. Use this same basis on all manufacturers’ performance charts and you will see that 4-5 GPM is pretty much the standard for residential models here in the Pacific Northwest.