If you add batteries in series the voltage will increase by 12v with every battery added (not ideal if you are running 12v only). In parallel the voltage will remain 12v but the available current will increase. Regular battery inspection and maintenance is a must. I have set up to run emergency lights and the TV/ decoder only from inverters / battery / solar panel recharging with two generators (2500w and 6500w) on standby if the power goes down for a long time (to keep refrigerators going). Not ideal but cost effective, and you can buy / add to the picture as funds become available.