Two years ago I spent a week in a passive house near Piteå. The outside temperature was -25c and heating the house was done exclusively using an accumulation tank of solar heated water (yes. It worked just fine in -25 if the sun was shining for more than 2 hour a day).

The only problem was that it got too hot when we were making food or running the tumble dryer.

Heating for that house was between 500 and 2000 kwh/yr depending on how much the sun was shining.