I think there are a lot more folks here who have experience with the gas tankless heaters. Thus the lack of response.

The main issue w/ electric on-demand heaters is the massive amp draw to heat the water. I did a quick look, and one unit that'd do 2 gpm at a 60 degree F temperature rise (maybe enough for your shower and sink in the winter) takes 80 amps to do that. That might require a service upgrade that'd be pretty painful financially. And I'm not sure if you can run a conductor that big inside your walls without conduit as protection.

You've probably read that gas heaters have their own issues. The equivalent one is the need for a large gas feed (and possibly a new meter to meet the demand). But that's relatively low-tech piping that has to be run. Ditto on the need for expensive vent piping if you put one inside. And don't forget that, if your power goes off, no hot water.

I guess there's some potential to mount a gas tankless on an outside wall, which is what we did with our Noritz. That gets rid of some of the issues, but then you need to make sure none of that piping is going to freeze. I'm in GA, and piping in outside walls makes me nervous here. That'd increase by a big amount for the DC area.

There are lots of pros/cons/tradeoffs. Putting a bigger (and preferably more efficient) tank heater in your basement likely is going to be the cheapest option. Going to either type of tankless will save you on waiting for hot water, and it may save a little on energy usage, but that's going to come at a significant additional cost.

Thull, thanks for the input. We already have a good gas water heater that is just 4 years old. The issue isn't running out of hot water, the issue is having to wait a long time for hot water (which we already do in our second floor bath, I imagine on the third floor it would be worse), and losing hot water in the middle of a shower when our tenant happens to run water at the same time (and in the morning when we're all getting ready for work, that's fairly often).

Ideally we would have 2 separate water heaters, one for us and one for our tenant. Unfortunately the plumbing was done before we bought the house, it would require work on the apartment that we don't want to do, and squeezing another water heater into the boiler room in the basement would be difficult.

The amperage rating of an electric tankless water heater is what it will pull at maximum output. The electric supply must be able to meet that demand, but the unit WILL NOT ... I repeat, WILL NOT use that much current every time it runs. Electric tankless nowadays is engineered to throttle its power demand according to the incoming temp, flow rate, and setpoint (outgoing temp). My tankless is rated 120 amps at 240 volts. It sometimes runs low as 5% to 15% of that under some usage conditions. It typically averages between 30% and 60% (60% of 120 is 72). I have seen it reach 85% to maximum in rare cases, such as blasting a load of white laundry at 140°F during a cold snap in the winter season.

johnnieb- sounds to me like you have a pressure or line size problem; maybe one of the real plumbers will chime in and offer some suggestions. There are a variety of ways you could do recirculation, but most require extra piping, and you'll be running that pipe like a nice radiator in your wall.

dadoes- even if it isn't running full bore, you still have to size the electric service based on a percentage of the connected load; there's a decent chance that adding the electric tankless will push you into a larger size service, especially if what you have is older (100A, 150A, e.g.).