I assume you are referring to the Solar Cell block in SimElectronics. What are you trying to connect it to? If you look at the documentation, you'll see that it has 2 electrical ports from the Simscape foundation domain. You can therefore connect it to any Simscape or SimElectronics component that has electrical ports. The physical signal input to the block is the incident irradiance. You can use any Simulink signal for that, which you'll need to convert into a physical signal using the Simulink-PS Converter block.