You have some great questions here. Your project sounds like a great idea and it is something that is definitely a hot topic right now with solar energy gaining popularity. The good thing is that you can test whether a battery or whether a solar panel is more effective at producing energy.
Identifying a testable question is the first step in the "Scientific Method." View the process here: http://www.sciencebuddies.org/science-f ... ificmethod
So before really deciding what project to do, decide what your question will be. If you have a question, it can be easier to look up projects that will help you answer your question. Some examples of good questions can be:
1. Which provides a longer energy source, a battery or solar-cell?
2. How do solar cells work?
3. How long can a car run using solar energy?
Here is a project you might want to check out about how batteries work, and it is pretty inexpensive:http://www.sciencebuddies.org/science-f ... background
Also, I have a couple questions:
1. Are you building the robot yourself, or do you have one already built (i.e. from a package)?
2. What is it about his project you are interested in? Meaning, are you more interested in robotics or the way that solar energy works? It is important to identify what kind of information you are trying to find out. Learning how robots function and the efficiency of batteries vs. solar energy are different projects. It might be a good idea to check some different project ideas on this website: here are some ideas for your grade level on electricity.http://www.sciencebuddies.org/science-f ... ediate&p=1
If you want to compare solar vs. electrical energy, you can use insulated alligator clips/ wires attached to the battery (typical or solar) and the other end connected to a lightbulb.http://astore.amazon.com/sciencebuddie- ... B000F5IHHS
Here are some inexpensive tools you might need for testing the effectiveness of the different batteries:
This is a solar cell you can buy at Radioshack for about $5: http://www.radioshack.com/product/index ... 1#inTheBox
If you are interested in building a solar powered car, here is a website that tells you how and what you need:http://www.xof1.com/buildMiniSolarCar.php
There are other things I can help with, but check out those resources and let us know what you think and where you want to go from there. It sounds like you will have a lot of fun with this topic. I can't wait to see what you come up with.