diff --git a/core/metrics/collector.go b/core/metrics/collector.go
index 1d829f786..1b9c29216 100644
--- a/core/metrics/collector.go
+++ b/core/metrics/collector.go
@@ -56,19 +56,32 @@ func (c *Collector) process(fun func()) error {
 
 	fun()
 
-	if slotStart := now.Truncate(tariff.SlotDuration); slotStart.After(c.started) {
-		// skip incomplete first slot
-		if !c.started.IsZero() {
+	slotStart := now.Truncate(tariff.SlotDuration)
+
+	switch {
+	case c.started.IsZero():
+		// keep started un-truncated so a mid-slot start stays distinguishable
+		c.started = now
+
+	case slotStart.After(c.started):
+		// persist the completed slot only if started is the immediately
+		// preceding slot boundary - false for the mid-slot first slot and
+		// for a slot reached after a data gap
+		if c.started.Equal(slotStart.Add(-tariff.SlotDuration)) {
 			if err := c.persist(); err != nil {
 				return err
 			}
 		}
 
 		c.started = slotStart
-		c.accu.Energy = 0
-		c.accu.ReturnEnergy = 0
+
+	default:
+		return nil
 	}
 
+	c.accu.Energy = 0
+	c.accu.ReturnEnergy = 0
+
 	return nil
 }
 
diff --git a/core/metrics/collector_test.go b/core/metrics/collector_test.go
index 6263af368..703179df4 100644
--- a/core/metrics/collector_test.go
+++ b/core/metrics/collector_test.go
@@ -154,3 +154,40 @@ func TestCollectorSetImportAndExportMeterTotal(t *testing.T) {
 	require.InDelta(t, 0.3, col.accu.Imported(), 1e-10)
 	require.InDelta(t, 0.7, col.accu.Exported(), 1e-10)
 }
+
+// TestCollectorSkipsPartialFirstSlot verifies that the first slot, joined
+// mid-way after (re)start, is not persisted as a full 15min slot.
+func TestCollectorSkipsPartialFirstSlot(t *testing.T) {
+	clk := clock.NewMock() // 1970-01-01 00:00:00 UTC
+
+	require.NoError(t, db.NewInstance("sqlite", ":memory:"))
+	require.NoError(t, SetupSchema())
+
+	col, err := NewCollector("partial", "partial", WithClock(clk))
+	require.NoError(t, err)
+
+	// first update mid-slot (00:05) - slot 00:00 is only partially covered
+	clk.Add(5 * time.Minute)
+	require.NoError(t, col.AddEnergy(nil, nil, 1e3))
+	clk.Add(5 * time.Minute) // 00:10
+	require.NoError(t, col.AddEnergy(nil, nil, 1e3))
+
+	// cross into slot 00:15 - the partial slot 00:00 must not be persisted
+	clk.Add(5 * time.Minute) // 00:15
+	require.NoError(t, col.AddEnergy(nil, nil, 1e3))
+
+	var count int64
+	require.NoError(t, db.Instance.Model(new(meter)).Count(&count).Error)
+	require.Zero(t, count, "partial first slot must not be persisted")
+
+	// cross into slot 00:30 - the fully covered slot 00:15 must be persisted
+	clk.Add(15 * time.Minute) // 00:30
+	require.NoError(t, col.AddEnergy(nil, nil, 1e3))
+
+	require.NoError(t, db.Instance.Model(new(meter)).Count(&count).Error)
+	require.EqualValues(t, 1, count, "first full slot must be persisted")
+
+	var m meter
+	require.NoError(t, db.Instance.First(&m).Error)
+	require.Equal(t, int64(15*60), m.Timestamp, "persisted slot should start at 00:15")
+}