If you know your AC draw, you can calculate AC amps and AC volts. Load will determine current. Volts times amps divided by .8 gives you watts on the DC side. The reason that you are dividing by .8 is to conservatively take into account the efficiency of the inverter. Dividing by .8 will provide the DC watts for the worst case scenario.
Take the result and divide by DC volts and that will equal the DC amps.
Here's an example.
You want to know the DC amps for a 24 volt inverter supporting a 1000 watt load.
1000 watts is 120 (the volts) x 8.33 (AC amps); divide by .8 and that's DC watts.
The result is 1250 DC watts. Divide by 24 and you have 52.08 DC amps.
Note: There are other factors besides DC amps that determine the minimum cable size and maximum cable length. Please use the recommended cable sizes/lengths as detailed in the specific product user guides.