Since you have looked at what will an 800 watt inverter run, let us look at the battery capacity. So, this will be determined by the combined consumption of appliances connected to it and the battery capacities (Ah) used. 1. A 100Ah AGM or GEL battery with a discharge level of 50% will run. Let us calculate the number of batteries required for an 800 watt inverter. To find out how many batteries do you need for a 800 watt inverter, you have to know the hours for which your inverter runs and multiply this by the number of watts the inverter is capable of. How many batteries do I need for a 800 watt inverter is now known to you. An 800-watt inverter is capable of supplying AC power using a 12V (let's assume) to run light electronic devices and electrical tools. If you are running the inverter for 4 hours, then total power =.