So i slapped a Lora Bee on top for and placed it near a window for testing yesterday.
Lipo is 1000mah and the board reads it's onboard temp sensor and sends Temp, Lipo voltage and RTC every minute and then the STM goes to sleep. The Lora module is not put to sleep and so draws ~3ma while idle. Putting the Lora module and the STM to sleep would drop current down to the uA's while sleeping.
So overnight the battery drop 0.07v and was topped back up very quickly once the sun came out the play.
So with putting both the uC and the Lora module to sleep you could use a pretty small lipo and solar panel.