From your description it sounds like you have an electric water heater. If it's a 40+ gallon unit, there will be two coils that heat the water. When you find that you have some hot water, but not as much as you typically would, the problem is that one of those two coils is burned out. The coils are located behind access panels near the top and bottom of the water heater. Replacing one is usually a job for a plumber but you could do it yourself if you have the tools and the skills. The power has to be off and the tank drained to even begin the process. You also need to understand how to test each coil, using a continuity tester to determine which of the two is burned out.As for your question about tankless water heaters, there's no such thing as an energy efficient ELECTRIC tankless water heater. Tankless water heaters are designed more efficiently for use with natural gas or propane. However, you may want to look into a new type of water heater called a heat pump water heater such as the Rheem HP50, which is a far more efficient way to heat water than a standard electric water heater.