GPM (gallons per minute)
GPM measures how much hot water a tankless water heater can deliver continuously. A shower uses roughly 2 GPM, a bathroom sink 0.5–1 GPM, and a tub filler 4 GPM — a tankless must sustain the sum of everything running at once.
Unlike a tank, a tankless heater never runs out of hot water — but it caps how much can flow at one time. If the demand exceeds its capacity at the current temperature rise, the water gets cooler or the unit throttles flow.
ENERGY STAR's certified GPM is measured at the DOE test's 77 °F temperature rise, which is why it reads lower than the marketing "up to X GPM" figure — that claim usually assumes a mild 35–45 °F rise you'd only see in warm climates.