My understanding is DST did still save appreciable energy until we replaced incandescent lights with fluorescent and leds. Longer daylight in the evening when people are awake and less in the early morning when people are asleep means lights aren't being used as much. The average light bulb used to consume 60 watts or more and also let off significant undesirable heat, so with a house full of lights DST really did cut back energy usage. Now though with led lights low consumption and virtually no heat, it's not nearly as significant.