The capacity of a tankless heater that's significant is the temperature RISE: the amount it can raise the inlet water by the time it gets to the outlet, since the water just makes a pass through the heater, never sits in a tank. Ours is 95F, which is the best we've ever had (we've had 3 since 1977, one made in Britain one French, and this one's a Bosch Aquastar.....all propane. Since we're in the country, on well water that runs 55F year around, the math isn't difficult....but. The "but" is that these things are temperature limited. Given the performance of the heater, one would think it'd do 150F, but it's actually 140F. Why 140? Several reasons; first, that is the magic temperature at which hot water burns skin, second, the more you heat the less efficient things become, third, any and all household work can be done with water that hot, and these are designed as household heaters. You will not reach the magic sparge temp of 170F with any tankless heater I know of.
