by pebe » Tue Sep 02, 2014 6:48 am
Any device will only take the current that it needs. In your case, the two 12V devices will take 1A each. The 9V 0.6A device when connected to a buck converter fed from a 12V supply, will take about 0.45A. So the total current taken by all three devices will be 2.45A.
Any lead acid 12V battery (whether for UPS or anything else) will have an 'Ah' rating. That shows its capacity to store electricity, and is the product of how many Amps it can supply for a given number of hours. For example a typical UPS battery has a rating of 17Ah. That means it can supply 1A for 17hours, or 2A for 8.5hours, or 17A for 1hour, or any combination of amps and hours that equals 17.
In your case the current required is 2.45A, so a 17Ah battery could supply that current for 17/2.45 = 6.93hours before it became discharged.
I hope that helps.