If you use the Golden Rule of application-application-application
you do not start at cost, you start at need.
what is the power/current/voltage you need to SWITCH
and what is (are) the most available power sources that do not need alteration such as transformer or power supplies.
and of course what do you have to switch them.
if you are doing the EM ergency light, you only need a 120V coil and nothing else.
if you happened to look up emergency light schematics, you would have found it is just connecting the relay to the lines voltage and nothing else. an indicator light adds a nice touch.