Electric Water Heaters: Tank vs Tankless (Instant On) Energy Usage?
...in a modern tanked heater with modern levels of insulation, and people actually using hot water from time to time, standby losses are microscopic .vs. use. So a tankless electric is a very expensive electrical installation (huge power draw needs huge wiring and often a service upgrade to support something like 3X 40A breakers) that might save 1-3% on heating water.

If you want to save money on heating water, consider a tanked heat pump water heater - there you can get some serious efficiency that actually applies to the hot water you use. Unless you heat your house with electric resistance heat, it's a win even in the winter, (your heating source heats the air that the heat pump removes heat from to heat the water) and in the summer it helps keep the house cool "for free."