So a normal relay is OFF and when powered is ON. Remove the power to the coil and the relay switches OFF. Simple, right?
But during all the time the relay is ON, it is consuming power to keep that electromagnetic coil energized.
That's a waste of energy.
With 1 or 2 relays, it's not a big deal. Say each relay is 10mA. Total of 20mA. Not bad.
If you're using 5 relays, and the user engaged all 5 relays to be on, that's 50mA wasted power... on what? Nothing but powering these relays.
Now, there is a certain kind of relay called a LATCHING relay. Apply a short burst of voltage to the coil and the relay turns ON.... remove the power, and the relay STAYS in the ON position. There's the power savings! To turn the relay off, you can either apply a reverse voltage to the coil to UNLATCH it, or apply power to a secondary coil to LATCH it in the other direction (i.e. the OFF). After the relay switches to OFF, you can remove the power and it stays OFF.
Very nice from a power-saving point of view. Especially if you're looking into using A LOT of relays for a particular project.
The problem is the logic to control these LATCHING RELAYS. One solution involves the use of microprocessors... but connecting/using up all the microchip's pins to just set/unset latching relays is a waste.
Currently looking at relay driver chips that may be simpler/easier to control.