Need to know battery configuration for a 250 watts inverter
Ball park figure calculation.

Assuming an 80% efficiency 

              Power in = 100/80 x power out


power out will be 250W (maximum) then required power in will be about 313 watts

By Ohm's law this gives a figure of 313/12 Amps i.e. 26Amps

So if you need to run for 1 hour this would suggest you need a battery of at least 26 Amp hr rating.