How long will my inverter last with "X" number of batteries?

When choosing batteries for your battery bank, you should consider two things; what you are running and how long you want to run your system? 

Now you can take the output AC amps multiply them by the volts output (vac) to figure watts.  Then, watts divided by input volts (vdc) gives the DC amps consumed per hour.  Once you have your DC amps per hour, you need to include the power loss through the inversion, which is about 10%.  Generally you should just multiply your DC amps per hour by 1.1 and this will give you the right amount in your final DC amps required total.

Amps ac (your load) x Volts ac = Watts

Watts/Voltage dc = Amps dc per hour required

Amps dc per hour  x  1.1 = actual Amps dc