What is an efficient method to detect the presence of Line AC power in an isolated method?
My first thought would be to use an optocoupler, but in order to do that, you need to supply the current needed. Placing a diode in series would make it only activate on half of the signal and save power. For example if you need 20mA to drive the optocoupler you would use resistors to limit the current. When you multiply 20mA by 120 or 240V you get 1200 or 2400 mW which is not efficient.
What other methods can be used to detect the presence of the voltage in an more efficient manor without using a step down transformer to get a lower voltage? My goal would be to get a circuit that uses limited power and common components.
I am sure someone is going to try and mark this as a duplicate of the many other 120VAC detection questions, but in this one I am looking for a more efficient method while still achieving the isolation.
Also the reason I do not want to use a transformer is due to the cost and component size as most transformers are not meant for only a few mW.