Sounds good but then when that small tank ran out of hot water, I guess there would be a period of cold coming through until the main tank backed it up? Unless

Sounds good but then when that small tank ran out of hot water, I guess there would be a period of cold coming through until the main tank backed it up? Unless the small tank was big enough for most conceivable uses - in which case, e.g. in the bathroom, it would become a large tank. A continuous flow unit at least can do what you said in your earlier post, e.g. instantaneously heat up the cold until it starts coming through hot. That's why I was thinking the supercaps, they might provide just enough juice for those 45 seconds or whatever, and also hopefully be reliable enough for decades of use, like you would expect from a hot water appliance.