Your incoming water temperature — in winter. A tankless unit's flow rating depends entirely on how many degrees it must lift your incoming water. Water arriving at 70°F heated to 120°F is a 50-degree rise; the same target from 40°F in winter is 80 degrees. The same unit delivers considerably less flow at the higher rise. What a bad answer means: a system sized on a summer figure, an annual average, or a manufacturer's headline number performs beautifully for most of the year and disappoints in exactly the months you'd notice. Ask any bidder what rise they sized against.