Sheesh, that is a difficult one!
It's impossible to say, tbh, as every system is different. Your idea of a "light" load could be different from mine!
Your best bet would be to get someone to test the power consumption at FULL load, and work from that figure as obviously there will be times when that load will be used and it will chew the battery power of anything you design to be used at a lower power.
Oh, do remember to include your monitor in your power calculations. A PC ain't much use if you have nothing to look at!