You need to use 10 hours of the 20hr rate to get to 50%. the 20hr rate is just the battery AH rating divided by 20. EG 232/20 =11.6a or 100/20 =5a
the 20hr rate is different for each amount of AH capacity rating.
Now you want to know the time for different amps instead of the 20hr rate, starting with knowing the watts.
Now it gets sort of ugly. The voltage of the battery keeps changing when it is being used , so the amps change if the watts stay the same.
If you use light bulbs though, they get dimmer when voltage drops, so their amps go down! To keep the same amps you have to keep turning on more lights as the battery goes down. :)
OTOH an inverter load will have amps creeping up as the inverter maintains the watts while voltage goes down.
So ignoring that ugly stuff, pretend you have steady amps and that battery voltage is your pick- a- number like 12.4v for your spreadsheet info. Now you have the amps to use for the various watt loads.
With those amps, you compare with the 20hr rate amps using Peukert (ok it just got ugly again) But--to make it easy to do Peukert, now you can go back to that US battery table linked above to see how that would work--not forgetting about you going to 50% so 10 hrs worth instead of going 20hrs to flat.
