Depends on what you define as “very cold”. Comparatively few people live in climates where you consistently have extended periods where the average temperature

Depends on what you define as “very cold”. Comparatively few people live in climates where you consistently have extended periods where the average temperature drops below zero. And there are ways to work with that. For example what’s in German called “Eisspeicherheizung” (ice storage heating). It cleverly uses the crystalization energy of water [1]. You basically bury a few tons of water and use it as a buffer. You drive a heat pump that extracts energy from the water and on the other side, you run a collector circuit that collects solar energy and heat from the air. Effectively, every time the temperature raises above 0 degrees, you fill up your buffer. The size of the buffer is designed so that at the end of the heating period, most water has been turned to ice. And now you also have free cooling for the summer. This system can cover periods where the outside temperature drops below zero without loss of efficiency. [1] freezing a liter of water at 0 degrees requires an equivalent energy to heating a liter by 80 degrees Celsius.