The basic operational principle of the battery-based solar PV system can be elaborated as follow. During the daytime, the PV panel generates electrical energy and the battery is used to store this portion of energy (line 1 in Figure 1). During the night, the energy is drawn from the battery to feed to the load (line 2 in Figure 1). Without the sunlight, the performance of the system depends solely on the capacity of the battery. The bigger the battery, the longer the system can operate without sunlight. This is known as autonomy, referring to the length of time in hours or days for which the battery-based solar PV system can operate without sunlight.
The autonomy can be calculated using the maximum capacity of the battery. For example, 3 days of autonomy means that the solar lighting system can operate for 3 days without the need to recharge the battery. However after 3 days the battery will be empty. If the system cannot be recharged due to overcast sky on the next day, the solar lighting unit will not operate since the energy stored in the battery is completely discharged during the 3-day autonomy.