Is it possible to power up Arduino upon serial transmission without dropping data? I have radiator valves that send readings about the temperature and valve opening to the serial output: Honeywell HR-25.

I want to collect those readings in a central server at home without flashing the thermostat with a custom firmware.

To do that, I need an Arduino (very bare one) that can receive the serial data, but at the same time I need it to be powered down most of the time, or it would kill the battery life of the thermostat.

Can I wake up an Arduino from power down using the serial transmission, and immediately capture the serial data without dropping bits? how?
I never tried myself, but afaik you can define an wake up/interrupt from USART, meaning that if you get a receiving byte from the UART it will wakeup the Arduino.

When powered up, you can read continuous data. I would be surprised if they make an UART wakeup signal, that bytes would be lost (i.e. that powering up would take too long to miss more UART bytes).