some hints

code/modules/power/apc.dm

@@ -290,6 +290,13 @@
 		set_machine_stat(stat | MAINT)
 		update_appearance()
 		addtimer(CALLBACK(src, .proc/update), 5)
+	register_context()
+
+/obj/machinery/power/apc/add_context(atom/source, list/context, obj/item/held_item, mob/living/user)
+	. = ..()
+	if(operating)
+		LAZYSET(context[SCREENTIP_CONTEXT_ALT_LMB], INTENT_ANY, locked ? "Unlock" : "Lock")
+		return CONTEXTUAL_SCREENTIP_SET
 
 /obj/machinery/power/apc/Destroy()
 	GLOB.apcs_list -= src