diff --git a/core/site_tariffs.go b/core/site_tariffs.go
index fca040df6..537b5924e 100644
--- a/core/site_tariffs.go
+++ b/core/site_tariffs.go
@@ -72,17 +72,21 @@ func accumulatedEnergy(rr timeseries, from, to time.Time) float64 {
 			continue
 		}
 
-		start := last.Timestamp
-		if start.Before(from) {
-			start = from
+		x1 := last.Timestamp
+		y1 := last.Value
+		if x1.Before(from) {
+			x1 = from
+			y1 += float64(from.Sub(last.Timestamp)) * (r.Value - last.Value) / float64(r.Timestamp.Sub(last.Timestamp))
 		}
 
-		end := r.Timestamp
-		if end.After(to) {
-			end = to
+		x2 := r.Timestamp
+		y2 := r.Value
+		if x2.After(to) {
+			x2 = to
+			y2 += float64(to.Sub(r.Timestamp)) * (r.Value - last.Value) / float64(r.Timestamp.Sub(last.Timestamp))
 		}
 
-		energy += (r.Value + last.Value) / 2 * end.Sub(start).Hours()
+		energy += (y1 + y2) / 2 * x2.Sub(x1).Hours()
 
 		if !r.Timestamp.Before(to) {
 			break
diff --git a/core/site_tariffs_test.go b/core/site_tariffs_test.go
index 52b621da6..6a4a78c6a 100644
--- a/core/site_tariffs_test.go
+++ b/core/site_tariffs_test.go
@@ -27,10 +27,14 @@ func TestAccumulatedEnergy(t *testing.T) {
 		expected float64
 	}{
 		{0, 0, 0},
-		{0, 0.5, 0.25},
+		{0, 0.5, 0.125},
 		{0, 1, 0.5},
-		{1, 2, 1.5},
+		{0, 1.5, 1.125},
 		{0, 2, 2},
+		{1, 2, 1.5},
+		{0.25, 0.75, 0.25},
+		{0.5, 1, 0.375},
+		{0.5, 3.5, 6},
 	} {
 		t.Logf("%d. %+v", i+1, tc)
 
@@ -38,6 +42,6 @@ func TestAccumulatedEnergy(t *testing.T) {
 		to := clock.Now().Add(time.Duration(float64(time.Hour) * tc.to))
 
 		res := accumulatedEnergy(rr, from, to)
-		assert.Equal(t, tc.expected, res, "test case %d", i)
+		assert.Equal(t, tc.expected, res, "test case %d", i+1)
 	}
 }