Students explore energy efficiency, focusing on renewable energy, by designing and building flat-plate solar water heaters. They apply their understanding of the three forms of heat transfer (conduction, convection and radiation), as well as how they relate to energy efficiency. They calculate the efficiency of the solar water heaters during initial and final tests and compare the efficiencies to those of models currently sold on the market (requiring some additional investigation by students). After comparing efficiencies, students explain how they would further improve their devices. Students learn about the trade-offs between efficiency and cost by calculating the total cost of their devices and evaluating cost per percent efficiency and per degree change of the water. This engineering curriculum meets Next Generation Science Standards (NGSS).