If you know ahead of time that you are going to use it like this, you can use a purpose built Thermal Store. It's a huge insulated water tank that you heat up, and then you extract heat from it by running mains water through a heat exchanger with the tank water. Typically used by people with wind, solar, or burned fuel energy supplies to smooth out energy consumption and soak up extra energy when there is too much. Also by people with variable electricity pricing to soak up free excess energy from the grid at night and heat their home for free all day.