The dirty little secret about heat pumps is that, when the outside ambient temperature gets low enough, they can’t gather enough heat to keep the indoors warm, so they switch over to electric resistance elements. This was a major contributing factor in the widespread grid breakdown in Texas last year.
Another unintended consequence of the greenies’ oh-so-altruistic policies.
It’s not a secret and the heatpump data is printed right in their design characteristics. Sometimes right on a sticker on top of the box. It’s a matter of proper engineering based on recorded climate data.
All heating systems (including fossil) are “only” designed for 99.6% of recorded conditions. And the grids maybe for 99.9%. Including gas mains which can only maintain pressure up to a certain demand.
It becomes incredibly cost prohibitive and in many cases impossible to design large systems for more extreme conditions than that.
Everyone that needs heat 100% of the time should have backups to not depend on the grid during those rare 0.1% conditions. Propane, kerosene, wood, etc. It should be common sense but people these days are bad at maths...