A battery backup system allows you to store energy while the sun is shining and use that energy at night or on cloudy days. Instead of giving the excess energy produced back to the grid and buy it back from the grid at a higher rate during night, it can be used to charge the battery for later usage. It is a popular way to save on energy bills. Another advantage of having a battery backup power system is that it provides you with a greater degree of energy independence. It becomes an interruptible power system (UPS), so that if the electricity supply is interrupted your system automatically switches to the battery supply for continuous power.