Always call update_use_power() instead of directly setting the use_power var.

- This will let us react to the change appropriately.
- While we're here, lets define some constants so we can stop using bare numbers.
- Always use those constants when calling update_use_power()

code/modules/shieldgen/emergency_shield.dm

@@ -156,7 +156,7 @@
 	idle_power_usage = 0
 	for(var/obj/machinery/shield/shield_tile in deployed_shields)
 		idle_power_usage += shield_tile.shield_idle_power
-	update_use_power(1)
+	update_use_power(USE_POWER_IDLE)
 
 /obj/machinery/shieldgen/proc/shields_down()
 	if(!active) return 0 //If it's already off, how did this get called?
@@ -166,7 +166,7 @@
 
 	collapse_shields()
 
-	update_use_power(0)
+	update_use_power(USE_POWER_OFF)
 
 /obj/machinery/shieldgen/proc/create_shields()
 	for(var/turf/target_tile in range(2, src))

code/modules/shieldgen/shield_diffuser.dm

@@ -57,7 +57,7 @@
 		update_icon()
 		return
 	enabled = !enabled
-	use_power = enabled + 1
+	update_use_power(enabled ? USE_POWER_ACTIVE : USE_POWER_IDLE)
 	update_icon()
 	to_chat(usr, "You turn \the [src] [enabled ? "on" : "off"].")