I spent a lot a lot a lot of time researching the benefits and downsides of tankless heaters because our 30 gallon tank is 19 years old and about to kick the bucket. If I were to ever get one, it'd be NG/Propane, not electric. I never will, since I plan on getting rid of my propane as is.
The electric tankless heaters require a lot of power, which may require upgrading your panel, and in some cases your transformer outside if you get a large enough.
Here's some typical hot water flow rates:
Dishwasher 0.5 - 1.0 GPM
Bathroom Sink 0.5 - 1.5 GPM
Kitchen Sink 1.5 - 2.0 GPM
Washing Machine 2.0 - 2.5 GPM
Shower 1.5 - 2.5 GPM
Bath Tub 2.5 - 4.0 GPM
Most likely you won't be running everything at once, but lets say 2-3 at once, shower, kitchen sink 3gpm, wife accidentally turns on the washing machine you're up to 5gpm (minimum).
If you run a middle of the line 240v 80amp heater it produces roughly 4.25GPM. .75 below your 5gpm according to the scenario above. But, here's the main thing to research, is the
Temp Rise. That 4.25gpm is based on a 30F temperature difference between your cold water in, and your set hot water temperature. If you set your hot water at 120 degrees (like a hot water tank), the 4.25GPM cold water temperature has to be 90 degrees to maintain 4.25gpm. No one's water comming in is that warm. Now, lets say cold water in is it's 60 degrees in the summer, that's a 70 degree temp rise. Your GPM is now reduced to 2GPM. This is a slightly low to medium sized, 80amp tankless heater. Which requires a minimum of 125AMP panel. So imagine winter, 45 degree incomming water temp, another 15 degrees to warm up.
These are absolute examples, obviously you're not running straight 2.0GPM 120 degree hot water while taking a shower, you're mixing in cold to bring the temp to something comfortable.
The tankless vs tank efficientcies are really usless too, in reality they're based on the most bottom of the line tank heaters, and even then, the time it will take to recoop the cost of a tankless heater, will require the lifetime of that unit to realize. I'm not against tankless, they're a great idea, especially if you don't have the room for a 40 gallon tank. But I would suggest a gas unit, unless your home is pretty small, and not a wife, 2.5 kids, and if you live somewhere fairly warm.
Unless money is not an issue, and you get a 150amp or better tankless unit (300amp recommended panel). Even still, talking nearly 8GPM, with very warm incomming water, still only 3.5GPM with a 70 degree temp rise (winter time).
And as far as point of use, I would not get those unless you're building a house, and can run it's own dedicated line for each. A small 120v point of use is still around 16a, requiring a 20amp breaker.
Not saying they're bad, I'd like to have one, if I was rich, and building a house from scratch, I would
Oh and if you were thinking gas anyway, then most of what I said is usless. But things to keep in mind with gas is still the temp rise, research the BTUs for your required GPM. You may perhaps even need to upgrade your line from the gas meter or tank to supply enough gas.
Oh and my real world exp with this, my sister in laws house, small 2bdrm, 1 bath. Had a small 50amp unit someone installed before she bought the house. They didn't upgrade their main panel, it was as old as the house, 100amp panel, she litterally couldn't use anything while taking a shower, perhaps a few lights. She asked me to check our her panel because it kept tripping the main breaker when she took a shower. Told her why. This was probably an example of someone tossing in a unit w/o knowing better to upgrade their electrical, but still. And we're in central Arkansas, with 112 degree summer heat, and winters not usually below mid 20s at most, for a very short time. Usually above freezing. And that 50amp she could only get by using just the shower, or just the sink, etc.
