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] On grid tie inverter is a device that converts the DC power output from the solar cells into AC power that meets the requirements of the grid and then feeds it back into the grid, and is the centerpiece of energy conversion and control for grid-connected photovoltaic systems.
[pdf] A specialized inverter receives power from your solar panels and converts the DC voltage they produce directly into grid-compatible AC power. The grid-tie inverter enables your home to not just import power from the utility, but export power to the utility as well.
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