I have a couple LED night lights that are triggered by a photocell to only turn on in the dark. I've often wondered if the trigger circuit consumed more power than just leaving the LED on.
It likely consumes next to no power: a proper switching MOSFET has negligible resistance when open, and has a very high gate resistance, so the photo cell does not have to produce any significant current. Possibly the current-limiting resistor in the power supply of the LED circuit dissipates more than the trigger circuit.
