About 1.2 watts to capture+process 1 frame/second - thats how much my current phone uses, and I suspect a cheaper platform will be more power efficient. You'd use a 5 watt solar panel (costing $4) in the southern USA, and a 10 watt panel (costing $9) in the northern USA. You would aim to rarely have spare power - in winter, you just drop the frame rate and the comms interval.

With more engineering effort, you can probably dramatically reduce power usage, for example by discarding parts of the frame that cars never drive through, which should allow extending the hardwares lifespan as batteries age and solar panels get covered in dust.