As a rule of thumb, if your 300W solar panel is rated at 24 Volts (nominal), and the distance between your solar panel and the solar charge controller is less than 25 feet (7.5m), a 12 AWG Copper wire would generally be sufficient.
[pdf] What is the normal current of a 30 watt solar panel? The normal current of a 30-watt solar panel typically ranges from 1.5 to 2.5 amperes, depending on various factors, including the specifications of the panel and environmental conditions. 1.
[pdf] The Government of St. Kitts and Nevis notified the public of a new agreement with the Republic of China (Taiwan) to develop a 50-megawatt land-based Solar PV and Battery Energy Storage Plant that will foster industrial growth and enhance energy resilience on the island.
[pdf] 
