I'd say the answers you got are pretty good. I doubt you will justify retrofitting one in place of a tank type. The plumbing and vent costs are too high on top of the cost of the unit. I do however use them on every home I build. I've had one at my house for 15 years.
Electric is a poor choice. The wattage required to match the performance of a gas unit is tremendous and can easily double the size of your electric service. If you want one that will make 6 gallons a minute at a 50 degree rise you may be unable to add the load to your existing electrical service. Check that out before looking further into an electric unit. I would never install an electric unit.
Tankless units typically use 190,000 btus per hour. That is a good bit of energy. A btu is the energy required to raise the temperature of one pound of water by one degree Fahrenheit. One cu ft of natural gas contains 1,000 btus of energy. My wife and I use around 20$ worth of propane to heat our water each month. The cost if we used natural gas would be 10$.
By comparison, to heat the same water with an electric tankless would go like this. First off the tankless of the same capacity would run about 48,000 watts which at 240 volts would be 200 amps. My electric heat pump that heats my house uses 10 amps when running by comparison. For that reason most electric tankless heaters for residential use compromise on capacity.
To heat 2,000 gallons of water from 70 degrees to 120 degrees would use 11 gallons of propane with a 82% efficient tankless heater. My cost is 20$. Natural gas would be 1,000 cu ft. Typical cost would be 10$ Electric would be almost 100 percent efficient. The same 2,000 gallons heated 50 degrees would require 243 Kwh of electric which at 10 cents per would run you $24.30 cents. The electricity cost isn't what gets you, it's the size of the service that would be required unless you are willing to limit your hot water use. Don't plan on taking a shower while using other hot water demands with a small electric tankless unit. Your shower will use 2 gallons a minute. That alone would require over 60 amps which is a 4 gauge wire.
My electric consumption at any one time has never exceeded 18 amps and I have 2 electric clothes dryers and electric heat. On laundry day we are running 2 washers, two dryers, a clothes iron, the dishwasher and the electric heat and a tv etc. I'm still only using less than a third of the energy of taking a shower with an electric tankless. The problem is, with heating water you need a lot of energy in a short period of time unless you store it and heat it over time.
I hope this helps.
