diff --git a/internal/server/handlers.go b/internal/server/handlers.go
index a2700f1..40cf4ce 100644
--- a/internal/server/handlers.go
+++ b/internal/server/handlers.go
@@ -268,7 +268,42 @@ func (s *Server) handleGetSensors(w http.ResponseWriter, r *http.Request) {
 		jsonResponse(w, []interface{}{})
 		return
 	}
-	jsonResponse(w, result.Sensors)
+	sensors := append([]models.SensorReading{}, result.Sensors...)
+	sensors = append(sensors, synthesizePSUVoltageSensors(result.Hardware)...)
+	jsonResponse(w, sensors)
+}
+
+func synthesizePSUVoltageSensors(hw *models.HardwareConfig) []models.SensorReading {
+	if hw == nil || len(hw.PowerSupply) == 0 {
+		return nil
+	}
+	const (
+		nominalV = 220.0
+		minV     = nominalV * 0.9 // 198V
+		maxV     = nominalV * 1.1 // 242V
+	)
+	out := make([]models.SensorReading, 0, len(hw.PowerSupply))
+	for _, psu := range hw.PowerSupply {
+		if psu.InputVoltage <= 0 {
+			continue
+		}
+		name := "PSU " + strings.TrimSpace(psu.Slot) + " input voltage"
+		if strings.TrimSpace(psu.Slot) == "" {
+			name = "PSU input voltage"
+		}
+		status := "ok"
+		if psu.InputVoltage < minV || psu.InputVoltage > maxV {
+			status = "warn"
+		}
+		out = append(out, models.SensorReading{
+			Name:   name,
+			Type:   "voltage",
+			Value:  psu.InputVoltage,
+			Unit:   "V",
+			Status: status,
+		})
+	}
+	return out
 }
 
 func (s *Server) handleGetConfig(w http.ResponseWriter, r *http.Request) {
diff --git a/web/static/css/style.css b/web/static/css/style.css
index 565663b..37362f5 100644
--- a/web/static/css/style.css
+++ b/web/static/css/style.css
@@ -662,6 +662,11 @@ code {
     border-left-color: #f39c12;
 }
 
+.sensor-card.voltage-out-of-range {
+    border-left-color: #e74c3c;
+    background: #fff5f5;
+}
+
 .sensor-card.ns {
     border-left-color: #95a5a6;
     opacity: 0.6;
diff --git a/web/static/js/app.js b/web/static/js/app.js
index 6f2413e..374cac2 100644
--- a/web/static/js/app.js
+++ b/web/static/js/app.js
@@ -1100,8 +1100,10 @@ function renderSensors(sensors) {
         items.forEach(s => {
             let valueStr = '';
             let statusClass = s.status === 'ok' ? 'ok' : (s.status === 'ns' ? 'ns' : 'warn');
+            const sensorName = String(s.name || '').toLowerCase();
+            const isPSUVoltage = type === 'voltage' && sensorName.includes('psu') && sensorName.includes('voltage');
 
-            if (s.value) {
+            if (Number.isFinite(s.value)) {
                 valueStr = `${s.value} ${s.unit}`;
             } else if (s.raw_value) {
                 valueStr = s.raw_value;
@@ -1109,7 +1111,13 @@ function renderSensors(sensors) {
                 valueStr = s.status;
             }
 
-            html += `<div class="sensor-card ${statusClass}">
+            // Highlight PSU mains voltage deviations from nominal 220V (±10%).
+            let extraClass = '';
+            if (isPSUVoltage && Number.isFinite(s.value) && (s.value < 198 || s.value > 242)) {
+                extraClass = ' voltage-out-of-range';
+            }
+
+            html += `<div class="sensor-card ${statusClass}${extraClass}">
                 <span class="sensor-name">${escapeHtml(s.name)}</span>
                 <span class="sensor-value">${escapeHtml(valueStr)}</span>
             </div>`;