From what I've read here it seems as if you are most concerned about triggering an alarm
when your system goes into an alarm
state. For this purpose the most power savvy relay is possibly no relay at all. Think of the alternator
light on your dashboard. The 'hot' side is wired to the ignition. The 'ground' side is connected to alternator
output. When alternator output is plus, there is no alarm. When it goes to zero, there is a path to ground and the light is illuminated. (There is often a diode to prevent alt light from glowing faintly when output is higher than ignition voltage.) in my setup, the alt light 'ground' is connected to the regulator
alarm terminal. In alarm state (not charging
, etc.) i get a light, which also triggers a buzzer.
Of course your alarm circuit could include a relay which is not energized until the ground side gets grounded (hence zero power draw), but you might not even need one, depending on the power needs of your alarm. The diode might still be a good idea, but I don't have one unless it is internal to my regulator