diff --git a/roles/prometheus/templates/alert.rules.yml.j2 b/roles/prometheus/templates/alert.rules.yml.j2
index c5ba1c2..e6f10ce 100644
--- a/roles/prometheus/templates/alert.rules.yml.j2
+++ b/roles/prometheus/templates/alert.rules.yml.j2
@@ -132,7 +132,9 @@ groups:
             Tension d'entrée de {{ raw('$value') }}V
 
       - alert: UpsWrongOutputVoltage
-        expr: (upsOutputVoltage < 220) or (upsOutputVoltage > 240)
+        expr: >-
+          abs(upsInputVoltage - avg_over_time(upsOutputVoltage[1d]))
+            < 3 * stddev_over_time(upsOutputVoltage[1d])
         for: 10m
         labels:
           severity: warning