Longman Dictionary of Contemporary English
storage heater
noun
EXAMPLES FROM CORPUS
▪ Check whether an off-peak electric storage heater or a gas fire might be better choice.
Wikipedia
Storage heater
A storage heater or heat bank (Australia) is an electrical heater which stores thermal energy during the evening, or at night when base load electricity is available at lower cost, and releases the heat during the day as required.
Usage examples of "storage heater".
The bed itself was an oasis of warmth and comfort, equipped with goose-down quilts, huge fluffy pillows, and Jamie, faithfully putting out British Thermal Units like an electric storage heater.