I assume we are not talking about a 120V or 220V solar panel that powers your house?
The maximum voltage you can connect to the Arduino is 5V.
For anything that's "safe", say below 40 or 50V, you can use a [u]Voltage Divider[/u] (2 resistors) to scale-down the voltage into the Arduino. You don't want the voltage divider to put a "load" on the solar panel, so I'd use resistor values that add-up to around 10k Ohms.