Ok, done.

code/modules/power/generator.dm

@@ -30,19 +30,15 @@
 	SSair.atmos_machinery -= src
 	return ..()
 
-/obj/machinery/power/generator/update_icon()
-
-	if(stat & (NOPOWER|BROKEN))
-		cut_overlays()
-	else
-		cut_overlays()
-
+/obj/machinery/power/generator/update_overlays()
+	. = ..()
+	if(!(stat & (NOPOWER|BROKEN)))
 		var/L = min(round(lastgenlev/100000),11)
 		if(L != 0)
-			add_overlay(image('icons/obj/power.dmi', "teg-op[L]"))
+			. += image('icons/obj/power.dmi', "teg-op[L]")
 
 		if(hot_circ && cold_circ)
-			add_overlay("teg-oc[lastcirc]")
+			. += "teg-oc[lastcirc]"
 
 
 #define GENRATE 800		// generator output coefficient from Q