So I have quite a bit of experience with electric whole house water heater:
Mine utilizes a 125amp breaker at the main, and 100 amp sub-panel. I have 400 amp service so this is not a problem for me. Older and smaller homes can have 100 - 200 amp service as standard, so this is your first area of research, as Don mentioned.
My tankless is smaller than a briefcase and built to be installed either in-wall or surface mount. Chamber material is important IMO so make sure you look for very high quality internal parts. I would also recommend a large high quality sediment filter mounted to clean incoming water to extend the life of your heater.
My unit measures incoming water temp. and only uses enough electricity to get the water up to your set outgoing temp. This is an upgrade option over many that are just full on when in operation. Because I can set the exact temp. the water leaves the heater, there is an added benefit that you are not heating water and then mixing it with cold at the tap to get the right temp. With tankless heaters, there is a correlation between volume and temp. so you can turn the volume down to get hotter water, whithin the set temp. range.
You are welcome to visit and see this unit in action. One major cautionary comment: These do not save money. While it may never need to be replaced in my life time, once family members get used to the idea that there is an endless supply of hot water, they tend to take very long showers. If you live by yourself, then you could probably realize a cost savings, and we did for the first couple of months. It took $25 off my monthly bill during the honeymoon phase.
I combined this install with appliances that heat their own water, and by purchasing a high end tankless heater, there is no real competition. Two people can shower while any of the other appliances are running, and that is a great convenience.
For those living in small homes, there is a space gain as well. Instead of a water tank closet, you could have another clothes closet or more room in the garage. Old style water heaters are designed to fail. Good quality tankless heaters are designed to last forever.
HTH!!