hotplate vs storage heater

hotplate

noun
  • A low-powered heating device for keeping food warm. 

  • The element of a stove. 

  • A semi-portable stove. 

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 hotplate and storage heater occurred in a corpus of books? (source: Google Ngram Viewer )