Replying to @TreasureDev

It depends on current draw though! For the red line, 500 mA draw, it can be 90% efficient at 3V, but ~95% efficient at 4.5V — so to get 500 mA, your gadget might need to pull (napkin math) 550 mA from two AA cells pushing 3V, but only 525 mA from three AA cells delivering 4.5 V.