3/4" gas feed sounds like I'd empty my 100 gallon tank pretty quick.
I really need to research volume usage.
Researching it is your best bet.
I know that since the tankless units do not try to keep water warm (like the tank units), and only fire up when hot water is demanded, they use less gas overall.
I also so no difference in time to get hot water to the showers over the tank unit, as it had to push the cold water out of the lines as well.
We may use more gas due to the fact I have the temperature set all the way up (about 160°). When water comes out of the kitchen sink it is scalding hot. That’s the way I like it. :)