This depends on the thermal insulation (my outside walls are 30cm think with mineral wool insulation) and quality of your house and windows to keep the heat inside. I have a ventilation unit that recycles the heat with a heat pump. Any additional heat is added as per design with electricity or for the extreme cold days I have a fireplace.

You're missing the point of the S9 as heater. I agree, if you have efficient heating in your house already, it's definitely not worth it. The point of the S9 is to replace an existing resistive heating source. Of course replacing a heat pump is not a good use for this. I live in Canada and as mentioned have $0.13/KWh (first 1350 KWh are at $0.095) rate. My house is kept warm with a wood stove and resistive heaters. An approved heat pump costs in the high thousands, close to $10k installed. Only 6-7 months of the year of use makes that investment damn near a decade to pay off, if it cuts my current bill by 70%.

But yeah, at $0.60/KWh, it makes way more sense to pay for heat pump install.