Generally speaking at least 7.5A per 100W of solar energy. e.g. if you want to install a 300W panel, then you need at least 22.5A charge controller. So just use a regular 30A charge controller, and you can add 100W panels later. Of course there is no problem to buy bigger ones, except for the money.

How to install the controller

First of all, always connect the solar panel at the end. Secondly, read carefully the charge controller's manual, which generally requires that

The controller should be installed as close as possible to the battery and leave enough space for ventilation. This will reduce the current loss. It can also help the controller with internal temperature sensors to accurately estimate the battery temperature for proper charging.

Do not install the charge controller above the battery. You do not want any gas to interfere with the controller's electronics.

Make sure to install the blown fuse close to the battery so that any system short circuit will not cause a fire.

What is the charge controller load output (LVD)

Most charge controllers have three sets of terminals. One set is for the battery. One set is plugged into the panel. Then, the third set usually shows LOAD or LVD.

The load output is intended to power all small electronic devices through the charge controller. One function of this output is that if the battery voltage drops back below a certain voltage, the charge controller will cut off the output to prevent further drain on the battery. Some smart charge controllers also adjust this voltage cutoff if your battery is not fully charged every day.

But this feature doesn't work that well because it's only available for low-power devices. You can't run an inverter through them.

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