I think you raise a very good point Engineer. Heat transfer for the holding tank option is solely by still air heat convection, which would be a very slow process as compared to a true HPWH. A small tank that is depleted daily will be less likely to reach room temperature than a larger tank that is barely depleted. The larger tank would take a longer time to reach room temperature, but barely using it would barely cause the temperature to deviate from room temperature.

Amazing! all those debates and in the end no real COP to speak off since nobody can without modeling tech & financial output reports to compare over some life cycle. "COP" is guessing used by those trying to debate or promote nothing.