u run into an extremely cloudy day where the solar panels don’t supply enough power to both run lights and recharge the battery system. Again you will use Ohm's law to determine if a second battery is necessary by calculating the total Kilowatt hours you plan to use each day and how many can be supplied by your system. Next check the Amp hours of your battery and multiply it by the voltage (in this case 12 volts), this will give you your total Kilowatt power for a full battery. Take the battery capacity and divide it by the daily usage to determine the highest run time of your system. Adding a second battery will double your run time.
Running 12 volt interior lighting is a great way to start to wean yourself off the grid. It also is a great back-up if you live in a location that experiences frequent power outages.