The input power usage will decrease in the same manner as the output power to a point.
There is a loss in the front-end due to the AC-DC conversion that becomes more significant as you reduce the output power. This will never exceed 10%.
WL4 40L EZ1 LP840 = 39.5 input watts
@ 50% = 19.75 watts + 10% tolerance = 23.7 watts
@ 10% = 3.95 watts + 10% tolerance = 7.9 watts