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When the panel is generating 12 Volts, C1 and C2 only need to be 2V (even a bit lower because of the diodes). At 24 Volts, this would be 4 Volt. If you use a standard 104 ceramic capacitor (rated at 16 Volt), you should be ok. BTW the capacitor is only used to make the voltage readout more stable for the arduino. Almost any value would suffice as long as it is rated above 4 Volt.