diff --git a/meter/sma.go b/meter/sma.go
index a60c3ca41..a3fd11a98 100644
--- a/meter/sma.go
+++ b/meter/sma.go
@@ -93,6 +93,33 @@ func (sm *SMA) waitForInitialValue() {
 	}
 }
 
+// update the actual meter data
+func (sm *SMA) updatePower(msg sma.Telegram) {
+	sm.mux.Lock()
+
+	if sm.powerO != "" {
+		// use user-defined obis
+		if power, ok := msg.Values[sm.powerO]; ok {
+			sm.power = power
+			sm.updated = time.Now()
+		}
+	} else {
+		sm.power = msg.Values[sma.ImportPower] - msg.Values[sma.ExportPower]
+		sm.updated = time.Now()
+	}
+
+	if sm.energyO != "" {
+		if energy, ok := msg.Values[sm.energyO]; ok {
+			sm.energy = energy
+			sm.updated = time.Now()
+		} else {
+			sm.log.WARN.Println("missing obis for energy")
+		}
+	}
+
+	sm.mux.Unlock()
+}
+
 // receive processes the channel message containing the multicast data
 func (sm *SMA) receive() {
 	for msg := range sm.recv {
@@ -100,29 +127,7 @@ func (sm *SMA) receive() {
 			continue
 		}
 
-		sm.mux.Lock()
-
-		if sm.powerO != "" {
-			// use user-defined obis
-			if power, ok := msg.Values[sm.powerO]; ok {
-				sm.power = power
-				sm.updated = time.Now()
-			}
-		} else {
-			sm.power = msg.Values[sma.ImportPower] - msg.Values[sma.ExportPower]
-			sm.updated = time.Now()
-		}
-
-		if sm.energyO != "" {
-			if energy, ok := msg.Values[sm.energyO]; ok {
-				sm.energy = energy
-				sm.updated = time.Now()
-			} else {
-				sm.log.WARN.Println("missing obis for energy")
-			}
-		}
-
-		sm.mux.Unlock()
+		sm.updatePower(msg)
 	}
 }
 
diff --git a/meter/sma_test.go b/meter/sma_test.go
new file mode 100644
index 000000000..363b591ac
--- /dev/null
+++ b/meter/sma_test.go
@@ -0,0 +1,49 @@
+package meter
+
+import (
+	"testing"
+
+	"github.com/andig/evcc/meter/sma"
+	"github.com/andig/evcc/util"
+)
+
+func TestSMAUpdatePower(t *testing.T) {
+	tests := []struct {
+		name      string
+		messsage  sma.Telegram
+		wantPower float64
+	}{
+		{
+			"success export",
+			sma.Telegram{
+				Values: map[string]float64{
+					"1:1.4.0": 0,
+					"1:2.4.0": 37.9,
+				},
+			},
+			-37.9,
+		},
+		{
+			"success import",
+			sma.Telegram{
+				Values: map[string]float64{
+					"1:1.4.0": 20,
+					"1:2.4.0": 0,
+				},
+			},
+			20,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			sm := &SMA{
+				log: util.NewLogger("sma "),
+			}
+
+			sm.updatePower(tt.messsage)
+			if sm.power != tt.wantPower {
+				t.Errorf("Listener.processMessage() got %v, want %v", sm.power, tt.wantPower)
+			}
+		})
+	}
+}