Bat Conservation International offers excellent information, and I'll link you to their bat house video that talks more about siting. In short however, BCI recommends mounting on a building rather than a pole but if the tree had significant mass, it might still be better than a pole -- The key is maintaining a consistent temperature, so that's why thermal mass is an issue. Furthermore, the bottom of the house should be at least 10' from the ground, but 12-20 is even better, and BCI recommends 20-30 from nearest trees. I encourage you to check out the Bat Boxes 101 video and the FAQs video and drop downs lower on the same page. https://www.batcon.org/about-bats/bat-gardens-houses/
Thank you for thinking of our bats!