immersion heater vs storage heater

immersion heater

noun
  • A device for supplying domestic hot water in which a thermostatically-controlled heating element is immersed within a tank of water 

storage heater

noun
  • An electrical heater which stores thermal energy during the evening or at night when electricity is available at lower cost, and releases the heat during the day as required. 

How often have the words immersion heater and storage heater occurred in a corpus of books? (source: Google Ngram Viewer )