Ummm... What do you want to do????
You could certainly build a solar enegry system, complete with a battery charger and a DC-to-AC converter, and even with automatic switching between solar & the power-company. and, that could be done without a microcontroller...
If you wanted to make a sun-tracking mirror, that would be a good Arduino project because you need something to "make decisions"... Do I need to move? What direction should I move? How much should I move? Have I moved far enough? Have I moved too far?
When I think about what makes compters (and microcontrollers, etc.) and programming useful, I think of the following things:
1. Doing things over-and-over in a loop. For example, making the same calculation in hundreds or thousands of cells in a spreadsheet.
2. Making calculations.
3. Making "logical" decisions. If the sun is out, and it's the weekend, turn-on the solar swimming pool heater. If it's the weekend, and the sun is behind a cloud, turn-on the gas pool heater. (But, one simple decision like that could be done without a "brain chip", although knowing what time it is and what day it is can be somewhat complicated, and it's easy for a computer.)
4. Storing data.