Wayne Whitney wrote:

. . . a well-insulated tank with a small gas burner can be more
efficient than a tankless with a larger burner for the same
capacity, measured in gallons per day.

Why do you say this Nick? Is the thermal effiency of a small gas
burner greater than the thermal efficiency of a large burner?

Maybe not. A 100K Btu/h tankless with 90% efficiency would waste 10X more heat
(10K Btu/h) than a tank with a 10K Btu/h burner with 90% efficiency, but it
might only run 10% of the time. Given the same size heat exchangers, the tank
heat exchanger with the lower firing rate would be more efficient.

I don't see anything wrong with the usual argument that a tankless
unit has greater overall efficiency because it has no standby losses.

IIRC, some tankless units have standby losses.

Nick