If you want to figure out how to get the correct size of electric water heater to suit your needs, what you need to do is make it bigger than what you actually want.
I'll give you an example. If you were going to have a 50 gallon electric water heater, you'd probably only get 60 percent of hot water out of that tank, so 30 gallons. The reason why you're only going to get 30 gallons is because as you're taking hot water out of the tank, you're putting cold water in it, and it dilutes.
Let's say for example, you have a big bathtub, and it takes 50 gallons. You're going to need an electric water heater, probably no smaller than 80 gallons and upwards of 100 just so you have enough hot water to fill that tank.
After you take the water out of that tank, then it's going to recover. It's probably going to be about 40 minutes to 45 minutes before that water gets hot again.