I have 3 batteries (AGM deep discharge) one is 35 amp, 55 amp and 105 amp.
My experience has been AGM batteries deliver 50% of the amp rating (in a sustainable way). You can take more amps out of the battery but it will take the voltage so low it will shorten the battery life. Do my 35 amp hour battery will only deliver 15 or 16 amps. Same for the others.
I measured this with a “cumulative amp meter” which totals the amps/watts extracted from the battery. I was using a 12 volt refrigerator as the load. The meter is great it only measures the current passing thru it (which factors in the on off cycle of the thermostat/compressor). I have used a regular in line amp/multi meter to measure but this meter is a big improvement. I don't have to sit there timing the duty cycle and off time etc.
Two examples of the meter I am using
http://amzn.to/2iWRGTl
This one seems to be a much better deal
http://amzn.to/2iWVW5e
I can almost squeeze 50% of the rated amps by taking the battery voltage down to 11.5 volts under load.
Would be nice it the battery ratings were for “useable amps”. I was fortunate to get the specs on my batteries which show the battery life for various loads and voltage drop graphs which is pretty close to reality. It’s worth looking for that information if you are shopping for a new battery.
Happy New Year
-Cosmo