What is the best cost effective mean(Power Consumption) to heat water for bath, Immersion Heater or Geyser?

What are the advantages of Immersion Heater Vs Geyser. By Immersion heater I'm referring to the ones we get in India which is immersed in a bucket. I think both of them consume electricity at very high rate. Which of these options is better and is there any other option like Solar Water Heater?