For a 24 volt system the panel at max power rating needs to be 32 to 36 volts. Roughly 16 to 18 volts for every 12 volts of battery. However that rule only applies if you are using a standard PWM or shunt regulator. Using that type of regulator you will loose 30% minimum of the power from the panels.
[pdf] The voltage output of a standard solar photovoltaic panel typically falls in the range of 18 to 36 volts. Monocrystalline panels commonly produce around 36 volts, while polycrystalline options usually output between 18 and 24 volts.
[pdf] Choosing the right battery voltage is key to designing an efficient off-grid system. The most common system voltages are 12V, 24V, and 48V. A 12V setup is typically used for small and simple systems, such as RVs, boats, or basic cabin setups.
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