This concept was widely deployed in certain regions with "storage heaters": very heavy (~40 Kg) electric heaters that would accumulate heat during the night whe

This concept was widely deployed in certain regions with "storage heaters": very heavy (~40 Kg) electric heaters that would accumulate heat during the night when electricity is cheap and release it throughout the day. They felt out of fashion because apparently they were hard to use as you cannot control how the heat is released. Charge them too much and you will have to open windows the next day wasting energy, charge them too little and you will have to turn them on during the day when the electricity is expensive.