Temperature rise
Temperature rise is the difference between incoming groundwater temperature and your hot-water setpoint. It determines how much flow a tankless water heater can actually sustain: the colder the groundwater, the fewer GPM you get.
Groundwater ranges from about 40 °F in the northern US to 75 °F in the far south. Heating to a 110–120 °F delivery temperature therefore means a rise of anywhere from 35 °F to 80 °F depending on where you live and the season.
The physics is fixed: GPM = BTU/h × efficiency ÷ (rise × 500). A unit that delivers 8 GPM at a 35 °F rise manages only about 4 GPM at 70 °F — winter in a cold state cuts tankless capacity roughly in half versus the brochure number.