This project first version uses a MCP73871 as Adafruits original solar design, and incorporates a INA219 to measure the current and power from the sun. Also, a fuel gauge is measuring the battery voltage and you can estimate how much time there is left for normal operation. In this way, you can better control sleep time for all of y'alls project. All data sent on I2C.
My high school students are going to use these in several solar science projects.