A historical usage graph from Ecobee. The line at the bottom represents outside temperature. In this period, it didn't break 0 Fahrenheit. We also see just five instances of the thermostat commanding auxiliary (backup) heat in the middle of the night, when temps dipped below -13
https://media.mas.to/masto-public/media_attachments/files/111/760/956/323/481/275/original/02d1835cd10c412d.png