A solar panel is rated 100W 12V. What would be the out-put of the same panel at 24V

you multiply volts times amps to get watts. So plugging in 24 volts to the equation means output is 50 watts in full sun.

Unless current stays constant, then the power output would double. Can you give us some more information? What exactly are you trying to do?

If you're talking about putting two of those together, you would still be at 12 V, but you would add the power to get 200W.

As I said, I don't quite understand what you're asking. Please clarify.