I tested a ~ 30W LED floodlamp buckboost charger and used my digital bench supply to measure input power from 9V to 16V with the internal battery pack near 12V and found as I expected that the worst efficiency dropped right around the battery voltage where it must have losses in both buck and boost with ripple decay noise in the ACDC Lithium charger. What I did was unplug the battery charger AC then charged up the battery externally to a normal Voltage range, then the charger no longer failed to trip out from over-current..
Outside that narrow battery voltage range, I found the charger efficiency was constant with the display on my bench supply as the brightness remained constant.
I found it on someone's curb, ready for garbage pickup or someone to pick for parts.
After I quickly discovered the failure due to battery being slightly undercharged and the over current shutdown tripping instantly, prevented it from operating, I knew how to fix it just by charging the battery externally. Now it operates on its own fine in both DC portable or AC, although I haven't done much stress testing on it. It now illuminates my garden.
All this anecdote proves is that this buck/boost charger was efficient above and below its operating range but significantly worse when the voltage gain was 1.0. We all know that losses occur from the square law of current thru any resistance.