The lowest power option depends on what you attach. With a poe injector you can use a dumb switch and a poe injector as an external device.
The poe injector has usually 3 connections: power and 2 ethernet jacks. One ethernet neck is for plain ethernet the other one has added poe. It loops through the data but adds power to one of the ports.
To determine the lowest power option or is useful to know your minimum specs. How many ports do you need in total? Do you need smart capabilities? Do you need or want morn than one poe port?
What's your solar capacity, and how much is left after you've powered your POE devices? You'll need a POE switch or injector that only needs that much. (Plus overhead and waste.)
You're probably better off scaling your solar array to fit the need than hunting for niche low power devices.