in order for a solar panel to safely charge your battery system, you need to have a voltage regulator that will output the appropriate voltage to your battery
if you are using lithium batteries, this regulator needs to be compatible with lithium batteries (many are NOT compatible, especially those which come with a solar panel as a kit)
these will generally require additional input/output power cables such as cables with Anderson plugs
consider using yellow plugs for the solar input circuit (athough some solar panels already are pre-wired with grey Anderson plugs or MC4 connectors) and standard grey plugs for the output to the battery circuit so you don't get them mixed up
yellow Anderson plugs will not plug into grey Anderson plugs!
there are two main options in solar regulators:
additional considerations:
MPPT design is around 10% more efficient than the older style PWM devices and will make better use of the solar panel
maximum current:
a 20A controller is adequate for 260W / 12V or 520W / 24V solar and for lithium batteries >= 20Ah; (AGM batteries must be >= 100Ah);
a 40A controller is adequate for 520W / 12V or 1040W / 24V solar and for lithium batteries >= 40Ah; (AGM batteries must be >= 200Ah);
maximum input voltage
the solar controller input max current should be at least 20% higher than the panel's short-circuit current
the solar controller input max voltage should be at least 4% higher than the panel's open circuit voltage.
size, weight and cost
ability to monitor battery temperature and reduce output if getting too hot
ability to send information via Bluetooth to smartphone or a remote meter
weathersealing