What are some of the biggest rewards of being a Solar Energy Systems Engineer?

First of all, realize that there is quite a range of "engineers". A Solar Energy Systems Engineer might be someone who recommends and sells solar energy systems. And their "reward" might be a commission. But let's assume you mean a real Systems Engineer -- someone who understands most or all of a system so as to make recommendations, modifications, enhancements, write requirements and architecture, etc.. And a Solar Energy Systems Engineer would be that person who does that for an application to take advantage of solar energy amplification -- i. e., providing "clean juice" for some application. The rewards of a Systems Engineer are studying a situation to see how to solve a problem like using solar for energy, understanding the customer's needs, whether the customer is aware of the or not, collecting information to come up with some solutions, communicating these in terms of requirements and architecture statements, shepherding the project along while working with implementers and designers, and seeing the result. The reward is achievement, and that achievement is often seeing people happy with the result. As a Systems Engineer, you're in the "crow's nest", so to speak, and in making people happy, that's a good place to be.

Hi Timothy

I would guess a great reward will be the current and likely long term very high demand for "Solar Energy Systems Engineer" and related skills. In the US the Dept of Energy has focus and opportunities in Solar Energy. Here is an example:

https://www.energy.gov/eere/solar/solar-energy-systems-designer

Other rewards would be participating in an energy transformation we most certainly are and will be experiencing for quite a while.