The document provide us details on the average power consumption on SRX 345.
Here is a documentation that would be useful for you -
https://www.juniper.net/documentation/us/en/hardware/srx345/topics/topic-map/srx345-site-guidelines.html
https://www.juniper.net/documentation/us/en/hardware/srx345/srx345.pdf
>>Average power consumption is 122 W