Well I'm afraid I must disagree with Kenneth. Using a 24V panel to charge 12v has benefits providing you are using a suitable MPPT solar charge controller. The easy way to see this in action is to experiment with this spreadsheet:
http://www.victronenergy.com/support-and-downloads/software#mppt-calculator-excel-sheet
Note you can also add panels of your own choice in the custom field. Higher voltages will be more efficient in terms of losses, the higher voltage also means you will also start charging earlier in the morning and continue later in the evening. For more information about matching panels to MPPTs see:
http://www.victronenergy.com/blog/2014/03/28/matching-victron-energy-solar-modules-to-the-new-mppt-charge-regulators/
If I had 12V batteries and say 2 x 12V panels of 50 Watts each I'd put them in series so 24V nominal @ 50 Watts instead of 12V nominal @ 100 Watts in parallel. Whilst the wattage for both (Volts x Amps = Watts) is the same for both the ones in series have less losses due higher voltage and less current. Try it in the sheet. Note most controllers will need to be +5V over battery voltage to start charging. That decides the number of cells you need in a panel to be sure you have a suitable voltage in the first place. Another reason to use higher voltages, plus the charge earlier, charge later i.e charge longer times overall as mentioned before.
Personally I found the sheet a very useful tool, along with the graphs, to see what is happening.
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