I am not sure that it makes sense to ever have an electric tankless hot water heater...
As James typed, besides all of the issues of installation--there is one big problem that I have only seen mentioned in one other place... That is the electric tankless water heaters control their temperature by rapidly cycling on and off... In many homes (if not all), causing the lights to dim and brighten repeatedly--very irritating.
I would humbly suggest to simply insulate the heck out of an existing electric hot water heater (including the water lines and T&P valve). Electric hot water heaters already have very high efficiencies (no central stack and pilot light like a gas hot water heater)--so a tankless one does not really save anything much.
And, if you can get Time of Use metering, you can heat your hot water off-peak and draw from the tank only during peak hours (for me, my summer peak/off-peak rates are $0.29/$0.09 per kWhr... The standard rate is $0.114 per kWhr. Winter peak/off-peak rates are $0.11/$0.09 per kWhr). My peak times (there are several TOU planes available) are Noon-6pm Monday through Friday. So--not too hard to avoid peak usage (I have gas myself). Cost me $277 for the meter and installation (there may be a one year minimum contract also--after that you can change back anytime if you wish).
There is a way of getting hot water for much less money (ignoring installation costs)... Some heat pump systems can also supply domestic hot water too.
And, you can go further and install a
geothermal heat pump (uses local water pond/river or buried in soil heat exchangers). Very popular in parts of the US right now (to the point that, from what I have read, that it is hard to interest a contractor in working on a residence right now--with all of the commercial work going on right now).
Lastly, if you have the ability to use Solar--then you might wish to look at solar hot water--In theory, solar hot water uses less roof space, costs less to install, and has a better return on investment than solar electric systems (I am looking at doing solar hot water--but I don't have a good place for the solar water panels right now, I already have installed a Solar PV Electric System--plus, many systems seem to demand quite a bit of maintenance and extra space—several reasons why I am still looking myself).
-Bill