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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.
I don't remember the exact statistics but around half of Finns have plan similar to mine where the prices change hourly. So far it has been cheaper on the long run but there are days when the prices are so high that you basically have to turn off heating or it'll cost you hundreds in one day. There is no upper limit to it but there's no lower limit either. Sometimes it goes to negative as well. Last year there was a day when it went so far into negative that people were earning money by running electric heaters outside in the winter.