For reference, the price for fixed-cost plans is around 10c/kWh.
As someone who’s been constantly running an electric heater in the garage while painting my car, I was quite lucky with the timing.
It’s not literally free, though. Transfer prices are fixed, and there are taxes and some other minor costs associated with it, so where I live, it still adds up to around 6c/kWh even when the price drops to zero. The cheap prices are due to an excess of wind power, but once the wind dies down, prices usually spike hard.