Those yard lights are so efficient because they are self contained. Short runs from charger to battery to light. You start pushing amps through wires and efficiency drops. Ohms are your enemy.
Yeah. but the interesting thing is that those things can actually be used to light up outdoor areas without even thinking about power. Costco had them on sale a while ago. $9.99 for box of 8. I thought they would be junk. So far they are really, really good. It surprised me, honestly.
But that is really a side issue.