Without getting too technical let’s take our setup of batteries in the caravan which is:
The key to a battery is the ah reading. Ah is short for Amp hours. So for simplicity reasons, a 100ah battery means you can use 1 amp of electricity from that battery for 100 hours before the battery is totally flat. Connecting batteries in series doubles the voltage but keeps the amp hours the same. Connecting batteries in parallel maintains the voltage and increases the amp hours. Refer to google if you want to know more but in general for caravans you’ll be connecting in parallel.
Having 3 x 120ah batteries connected in parallel gives us a total of 360ah of current we could use. however, a deep cycle battery does NOT like to go less than %50 charge or else the life expectancy of the battery drops dramatically. There’s a chemical reason for that which I won’t go into either but feel free to read the battery university website if your curious. Therefore our 360ah battery system is effectively 180ah "useable" amps. Which means in one day if we used 180amps of electricity, our batteries would be %50 flat. That’s as low as we would want them to ever be, which equals 7.5amps per hour usage.