I have a 50 gal electric hot water heater and after only one shower that is 20 minutes there is little hot water left for a second shower. We have to wait until the tank heats the water before the second. The house in approx 3 years old. Can you help?

One of your elements might be burned out or you don't have the water temperature set high enough or both.

If you have a heater that has a 4500 watt element that means that your heater will heat about thirteen gallons of water on a 100degree rise.

Most shower heads have a 3 gallon per minute flow so if you use the shower for 20 minutes that is 60 gallons of water, considering that half of that 60 is cold water then that would cut the hot portion down to 30 gallons, That would leave 20 gallons of hot water in the heater but as you are using the hot water out of the heater the water in it is getting cooler because the cold water is going to mix gradually no matter how long of a dip tube you have. So maybe cut the first shower in half.

You could turn up the thermostat a bit would help some.