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Solar panels can be wired to increase voltage or current. A normal panel's terminal voltage is rated between 17 and 22 Volts, but making use of a regulator regulates it to 13 Volts. The reason for this is that the safe voltage for charging a battery is between 13 and 14 Volts.

As mentioned, the solar panels can produce between 17 and 22 Volts. This is however a lot more than the safe range of between 13 and 14 Volts that you can charge a battery. To regulate this we make use of a solar regulators which drops the current causing a stable voltage.