How to best estimate power consumption for a system?
There is no simple answer.. Take a computer system with a 500W PSU.  The sum of all parts may be capable of dissipating 1000W but utilization due to usage may only be 100W on the average. The best way is to plan for more capacity and then cost reduce later after testing it.



It is much more complicated than your suggestion.  Current drawn depends on voltage, temperature . For sure you can estimate the nominal load current ought to be less than worst case load current.  Unless you know something about the load curent vs voltage and determine what the Power Fail threshold when your system should shut down safely, otherwise you can let it stop in unreliable ways. 

So you have no favourable measure of accuracy of predicting the operating time without experience or schematic and description to someone who understands. 

We also have no idea what your design is and if you have any  power fail circuit detection like the low voltage detection built into MOBO's

Sorry without more there is no simple answer to a software expert.  It would be like a hardware guy telling howe many lines of unique code gets executed per minute. You could look at all the code and features and multiply by some magic  utilization ratios. But the chance of it being accurate is inversely proportion to its complexity... Most like all you have to deal with is the power consumption of the top 3 items like the motors. But the the backup time is limited by the weakest link and we do not know if your power backup is balanced for each subsystem of batteries.