Students like the idea of solar power but they know surprisingly little about how it actually works.
I like to begin this unit with a short pretest that identifies students base knowledge on solar panels as it relates to 3 key ideas: the link between amount of light and energy produced, the angle of light and energy produced and the effect of heat or cold on energy produced.
Following this, I move on to a short series of lab stations that address the questions on the effect of angle and temperature and their impact on energy production. Both the pretest and the student lab directions are found in the Solar Power Cars - Pretest and Initial Testing document. The Solar Panel Investigations video describes the lab activities.
I purchased my kits from Kelvin; they have a lot to choose from so you can pick what best fits your needs. I do recommend purchasing at least one of the light sources for testing on overcast days as the solar panels from the kits do not generate enough power to move the cars with regular flashlights.
Students are instructed to use what they learned from the warm-up investigation to create a solar car that will complete against the other cars in the class. I remind students that because they all begin with the same kits, the difference in performance will come down to how they construct their vehicle. Students are instructed to use tape rather than glue when the can so they can alter their placement of parts if needed and only permanently affix components when they are certain of placement.
As I monitor group progress, I ask them questions to ensure they have considered items such as the pros/cons of light weight vehicles, the position of the sun during our class period, placement of the wheels to ensure straight motion, etc.
At some point during the "trial and error" time, students are likely to begin to become frustrated. This is a good time to implement a sharing strategy such as give an idea/get an idea. This strategy requires students to partner with another group, describe their current challenge along with what they have tried so far to address that challenge, and get some new ideas/suggestions from that other group and then flip roles. This is a great way to support students while reinforcing the value of collaboration and teamwork.
I provide students with 5-7 days to design-test-tweak-test depending on the complexity of the project and student need. I give them a projected "done by" date for final data collection to help them stay on task and motivated. I prefer that all students observe each group for the final testing so they can see how others approached the task and compare with their own design approach.